Clean up RTCVideoFrame

RTCVideoFrame is an ObjectiveC version of webrtc::VideoFrame, but it
currently contains some extra logic beyond that. We want RTCVideoFrame
to be as simple as possible, i.e. just a container with no extra state,
so we can use it as input to RTCVideoSource without complicating the
interface for consumers.

BUG=webrtc:7177
NOTRY=True

Review-Url: https://codereview.webrtc.org/2695203004
Cr-Commit-Position: refs/heads/master@{#16740}

webrtc/sdk/objc/Framework/Classes/RTCEAGLVideoView.m

@@ -215,12 +215,6 @@
 }
 
 - (void)renderFrame:(RTCVideoFrame *)frame {
-#if !TARGET_OS_IPHONE
-  // Generate the i420 frame on video send thread instead of main thread.
-  // TODO(tkchin): Remove this once RTCEAGLVideoView supports uploading
-  // CVPixelBuffer textures on OSX.
-  [frame convertBufferIfNeeded];
-#endif
   self.videoFrame = frame;
 }