MediaStream audio: Refactor 3 separate "glue" implementations into one.

content/renderer/media/media_stream_center.h

Index: content/renderer/media/media_stream_center.h
diff --git a/content/renderer/media/media_stream_center.h b/content/renderer/media/media_stream_center.h
index 5b72e6cd2bae68a28cc6e1bb40cd7f9ae33169aa..8b58afd3f39f8b1ff2d4b63045bdb439e94ad4ef 100644
--- a/content/renderer/media/media_stream_center.h
+++ b/content/renderer/media/media_stream_center.h
@@ -27,6 +27,8 @@ class PeerConnectionDependencyFactory;
 class CONTENT_EXPORT MediaStreamCenter
     : NON_EXPORTED_BASE(public blink::WebMediaStreamCenter) {
  public:
+  // TODO(miu): Remove these constructor args. They are no longer used.
+  // http://crbug.com/577874
   MediaStreamCenter(blink::WebMediaStreamCenterClient* client,
                     PeerConnectionDependencyFactory* factory);
   ~MediaStreamCenter() override;
@@ -58,10 +60,6 @@ class CONTENT_EXPORT MediaStreamCenter
       const blink::WebMediaStream& stream,
       const blink::WebMediaStreamTrack& track) override;
 
-  // |rtc_factory_| is a weak pointer and is owned by the RenderThreadImpl.
-  // It is valid as long as  RenderThreadImpl exist.
-  PeerConnectionDependencyFactory* rtc_factory_;
-
   DISALLOW_COPY_AND_ASSIGN(MediaStreamCenter);
 };