Revert of Stop caching supported codecs in WebRtcVideoEngine2 (patchset #1 id:1 of https://codereview.webrtc.org/2492473002/ )

Reason for revert:
This CL probably broke Chromium FYI.

Original issue's description:
> Stop caching supported codecs in WebRtcVideoEngine2
>
> We currently cache the result of GetSupportedCodecs in a member variable
> |video_codecs_| in WebRtcVideoEngine2. This means we need to keep
> |video_codecs_| and the result of GetSupportedCodecs in sync, which is
> error prone. It's simpler to just call GetSupportedCodecs when we need
> it, and we actually end up making fewer calls, so it's faster as well.
> This CL also returns all std::vectors by-value instead of by-ref. Move
> semantic together with in-place filtering of codecs actually end up with
> fewer copies, and it's also simpler to not return references.
>
> BUG=webrtc:6337
>
> Committed: https://crrev.com/9f71ec5a3e3175751f4475b126cfda89767363f2
> Cr-Commit-Position: refs/heads/master@{#15007}

TBR=tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6337

Review-Url: https://codereview.webrtc.org/2489173004
Cr-Commit-Position: refs/heads/master@{#15014}
diff --git a/webrtc/pc/channelmanager_unittest.cc b/webrtc/pc/channelmanager_unittest.cc
index 34b347b..174d064 100644
--- a/webrtc/pc/channelmanager_unittest.cc
+++ b/webrtc/pc/channelmanager_unittest.cc
@@ -173,17 +173,17 @@
   const VideoCodec rtx_codec(96, "rtx");
 
   // By default RTX is disabled.
-  codecs = cm_->GetSupportedVideoCodecs();
+  cm_->GetSupportedVideoCodecs(&codecs);
   EXPECT_FALSE(ContainsMatchingCodec(codecs, rtx_codec));
 
   // Enable and check.
   EXPECT_TRUE(cm_->SetVideoRtxEnabled(true));
-  codecs = cm_->GetSupportedVideoCodecs();
+  cm_->GetSupportedVideoCodecs(&codecs);
   EXPECT_TRUE(ContainsMatchingCodec(codecs, rtx_codec));
 
   // Disable and check.
   EXPECT_TRUE(cm_->SetVideoRtxEnabled(false));
-  codecs = cm_->GetSupportedVideoCodecs();
+  cm_->GetSupportedVideoCodecs(&codecs);
   EXPECT_FALSE(ContainsMatchingCodec(codecs, rtx_codec));
 
   // Cannot toggle rtx after initialization.
@@ -194,7 +194,7 @@
   // Can set again after terminate.
   cm_->Terminate();
   EXPECT_TRUE(cm_->SetVideoRtxEnabled(true));
-  codecs = cm_->GetSupportedVideoCodecs();
+  cm_->GetSupportedVideoCodecs(&codecs);
   EXPECT_TRUE(ContainsMatchingCodec(codecs, rtx_codec));
 }