Plumb audio muting state to guest web-contents.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 88f58a782fd7544ae316343b06dc0b6dd50546c2..bad723e85ce4c420ef77c62662530cb748362024 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -1120,6 +1120,10 @@ void WebContentsImpl::SetAudioMuted(bool mute) {
     audio_muter_->StopMuting();
   }
 
+  // If we have guests, they should have the same audio muting state.

[Charlie Reis, 2016/01/06 22:13:51]

If we use WebContentsObserver for this, it's general and we should omit the
comment (in case it gets stale).

On the flip side, if we just need to notify the guest WebContents(es), could we
iterate over inner_web_contents_tree_nodes_?  Or is that only populated in
--use-cross-process-frames-for-guests?

[wjmaclean, 2016/01/06 22:36:09]

I included the comment to explain who the (typical?) users of this notification
might be, but we can omit it. According to lfg@ inner_web_contents_tree_nodes_
is purely a WebView-over-OOPIF (Woo?) concept.

At present guests *are* the only consumers of this event, but that could
presumably change.

+  FOR_EACH_OBSERVER(WebContentsObserver, observers_,
+                    DidUpdateAudioMutingState(mute));
+
   // Notification for UI updates in response to the changed muting state.
   NotifyNavigationStateChanged(INVALIDATE_TYPE_TAB);
 }