Plumb audio muting state to guest web-contents.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index 5a55b72c971f4f6fde42b0698ba5bda60b6a9cff..3d770b73791e7f2c108a1707830aa750911e9a8a 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -397,6 +397,9 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
   // process.
   virtual void DidUpdateFaviconURL(const std::vector<FaviconURL>& candidates) {}
 
+  // Invoked when the WebContents is muted/unmuted.
+  virtual void DidUpdateAudioMutingState(bool muted) {}
+
   // Invoked when a pepper plugin creates and shows or destroys a fullscreen
   // RenderWidget.
   virtual void DidShowFullscreenWidget(int routing_id) {}