Bug 1061046. Part 1.5: Expose MediaStream::GraphRate(). r=karlt

This is guaranteed to work even if the stream has been disconnected from the graph,
so it's preferred to Graph()->GraphRate().

content/media/MediaStreamGraph.h

@@ -334,6 +334,11 @@ class MediaStream : public mozilla::LinkedListElement<MediaStream> {
   void SetGraphImpl(MediaStreamGraphImpl* aGraph);
   void SetGraphImpl(MediaStreamGraph* aGraph);
 
+  /**
+   * Returns sample rate of the graph.
+   */
+  TrackRate GraphRate() { return mBuffer.GraphRate(); }
+
   // Control API.
   // Since a stream can be played multiple ways, we need to combine independent
   // volume settings. The aKey parameter is used to keep volume settings

media/webrtc/signaling/test/FakeMediaStreams.h

@@ -71,6 +71,8 @@ class Fake_MediaStream {
  public:
   Fake_MediaStream () : mListeners(), mMutex("Fake MediaStream") {}
 
+  uint32_t GraphRate() { return 16000; }
+
   void AddListener(Fake_MediaStreamListener *aListener) {
     mozilla::MutexAutoLock lock(mMutex);
     mListeners.insert(aListener);