Reland "Don't create channel_manager++ when media_engine is not set"

This reverts commit c6c02efb56b24df04ed9ab61252c14c7bddcca93.

Reason for revert: Test now passes (and channel manager is gone)

Original change's description:
> Revert "Don't create channel_manager when media_engine is not set"
>
> This reverts commit c48ad732d6eb69f14dd6d44f801d62997cef2c2f.
>
> Reason for revert: breaks downstream project
>
> Original change's description:
> > Don't create channel_manager when media_engine is not set
> >
> > Also remove a bunch of functions in ChannelManager that were just
> > forwarding to MediaEngineInterface.
> >
> > Bug: webrtc:13931
> > Change-Id: Ia38591fd22c665cace16d032f5c1e384e413cded
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261304
> > Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> > Reviewed-by: Henrik Boström <hbos@webrtc.org>
> > Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> > Cr-Commit-Position: refs/heads/main@{#36801}
>
> Bug: webrtc:13931
> Change-Id: I1e260a2489547bd9483b50e043c28d2805b0fa5a
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261660
> Commit-Queue: Artem Titov <titovartem@webrtc.org>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Owners-Override: Artem Titov <titovartem@webrtc.org>
> Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#36811}

Bug: webrtc:13931
Change-Id: I7b5b45b46095c18d489b6a9fe4c625971d6b3da6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261661
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36976}
diff --git a/pc/media_session.cc b/pc/media_session.cc
index ec4b6a2..3f7dbb5 100644
--- a/pc/media_session.cc
+++ b/pc/media_session.cc
@@ -1566,9 +1566,7 @@
     const TransportDescriptionFactory* transport_desc_factory,
     rtc::UniqueRandomIdGenerator* ssrc_generator)
     : ssrc_generator_(ssrc_generator),
-      transport_desc_factory_(transport_desc_factory) {
-  RTC_DCHECK(ssrc_generator_);
-}
+      transport_desc_factory_(transport_desc_factory) {}
 
 MediaSessionDescriptionFactory::MediaSessionDescriptionFactory(
     cricket::MediaEngineInterface* media_engine,
@@ -2366,7 +2364,7 @@
   if (!CreateMediaContentOffer(
           media_description_options, session_options, filtered_codecs,
           sdes_policy, GetCryptos(current_content), crypto_suites,
-          audio_rtp_extensions, ssrc_generator_, current_streams, audio.get(),
+          audio_rtp_extensions, ssrc_generator(), current_streams, audio.get(),
           transport_desc_factory_->trials())) {
     return false;
   }
@@ -2478,7 +2476,7 @@
   if (!CreateMediaContentOffer(
           media_description_options, session_options, filtered_codecs,
           sdes_policy, GetCryptos(current_content), crypto_suites,
-          video_rtp_extensions, ssrc_generator_, current_streams, video.get(),
+          video_rtp_extensions, ssrc_generator(), current_streams, video.get(),
           transport_desc_factory_->trials())) {
     return false;
   }
@@ -2531,8 +2529,8 @@
 
   if (!CreateContentOffer(media_description_options, session_options,
                           sdes_policy, GetCryptos(current_content),
-                          crypto_suites, RtpHeaderExtensions(), ssrc_generator_,
-                          current_streams, data.get())) {
+                          crypto_suites, RtpHeaderExtensions(),
+                          ssrc_generator(), current_streams, data.get())) {
     return false;
   }
 
@@ -2673,7 +2671,7 @@
       audio_transport->secure() ? cricket::SEC_DISABLED : secure();
   if (!SetCodecsInAnswer(offer_audio_description, filtered_codecs,
                          media_description_options, session_options,
-                         ssrc_generator_, current_streams, audio_answer.get(),
+                         ssrc_generator(), current_streams, audio_answer.get(),
                          transport_desc_factory_->trials())) {
     return false;
   }
@@ -2681,7 +2679,7 @@
           offer_audio_description, media_description_options, session_options,
           sdes_policy, GetCryptos(current_content),
           filtered_rtp_header_extensions(default_audio_rtp_header_extensions),
-          ssrc_generator_, enable_encrypted_rtp_header_extensions_,
+          ssrc_generator(), enable_encrypted_rtp_header_extensions_,
           current_streams, bundle_enabled, audio_answer.get())) {
     return false;  // Fails the session setup.
   }
@@ -2809,7 +2807,7 @@
       video_transport->secure() ? cricket::SEC_DISABLED : secure();
   if (!SetCodecsInAnswer(offer_video_description, filtered_codecs,
                          media_description_options, session_options,
-                         ssrc_generator_, current_streams, video_answer.get(),
+                         ssrc_generator(), current_streams, video_answer.get(),
                          transport_desc_factory_->trials())) {
     return false;
   }
@@ -2817,7 +2815,7 @@
           offer_video_description, media_description_options, session_options,
           sdes_policy, GetCryptos(current_content),
           filtered_rtp_header_extensions(default_video_rtp_header_extensions),
-          ssrc_generator_, enable_encrypted_rtp_header_extensions_,
+          ssrc_generator(), enable_encrypted_rtp_header_extensions_,
           current_streams, bundle_enabled, video_answer.get())) {
     return false;  // Failed the session setup.
   }
@@ -2890,7 +2888,7 @@
     if (!CreateMediaContentAnswer(
             offer_data_description, media_description_options, session_options,
             sdes_policy, GetCryptos(current_content), RtpHeaderExtensions(),
-            ssrc_generator_, enable_encrypted_rtp_header_extensions_,
+            ssrc_generator(), enable_encrypted_rtp_header_extensions_,
             current_streams, bundle_enabled, data_answer.get())) {
       return false;  // Fails the session setup.
     }