[Media Router] Add Media Router Mojo impl code.

chrome/browser/media/router/media_sinks_observer.h

Index: chrome/browser/media/router/media_sinks_observer.h
diff --git a/chrome/browser/media/router/media_sinks_observer.h b/chrome/browser/media/router/media_sinks_observer.h
index 0b7bb5eb792dcbb17d050ad8784aca9f8241871e..36037edbef12834f727c31b2eadae3c9a33797d6 100644
--- a/chrome/browser/media/router/media_sinks_observer.h
+++ b/chrome/browser/media/router/media_sinks_observer.h
@@ -28,6 +28,9 @@ class MediaSinksObserver {
 
   // This function is invoked when the list of sinks compatible
   // with |source_| has been updated.
+  // Implementations may not perform operations that modify the Media Router's
+  // observer list. In particular, invoking this observer's destructor within
+  // OnSinksReceived will result in undefined behavior.
   virtual void OnSinksReceived(const std::vector<MediaSink>& sinks) {}
 
   const MediaSource& source() const { return source_; }