Let SessionDescription take ownership of MediaDescription

This documents in the API what is already true in the
implementation - that SessionDescription will eventually
delete MediaDescription objects passed to it.

The old API is preserved for backwards compatibility, but
marked as RTC_DEPRECATED.

Bug: webrtc:10701
Change-Id: I9a822b20cf3e58c5945fa51dbf6082960a332de8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/139880
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28144}
diff --git a/pc/webrtc_sdp.cc b/pc/webrtc_sdp.cc
index 96be3b2..af8a527 100644
--- a/pc/webrtc_sdp.cc
+++ b/pc/webrtc_sdp.cc
@@ -2798,7 +2798,7 @@
     desc->AddContent(content_name,
                      cricket::IsDtlsSctp(protocol) ? MediaProtocolType::kSctp
                                                    : MediaProtocolType::kRtp,
-                     content_rejected, bundle_only, content.release());
+                     content_rejected, bundle_only, std::move(content));
     // Create TransportInfo with the media level "ice-pwd" and "ice-ufrag".
     desc->AddTransportInfo(TransportInfo(content_name, transport));
   }