Add metrics regarding concurrent audible tabs in Chromium.

content/browser/media/media_web_contents_observer.h

Index: content/browser/media/media_web_contents_observer.h
diff --git a/content/browser/media/media_web_contents_observer.h b/content/browser/media/media_web_contents_observer.h
index 452349f4287f3083663e1bf88c2e52ca413b25a5..1f76029cf4e5d3b2a85376a7587ee58b0b4c3c7d 100644
--- a/content/browser/media/media_web_contents_observer.h
+++ b/content/browser/media/media_web_contents_observer.h
@@ -28,11 +28,11 @@ class CONTENT_EXPORT MediaWebContentsObserver : public WebContentsObserver {
   explicit MediaWebContentsObserver(WebContents* web_contents);
   ~MediaWebContentsObserver() override;
 
-  // Called when the audible state has changed.  If inaudible any audio power
-  // save blockers are released.
-  void MaybeUpdateAudibleState(bool recently_audible);
+  // Called by WebContentsImpl when the audible state may have changed.
+  void MaybeUpdateAudibleState();
 
   // WebContentsObserver implementation.
+  void WebContentsDestroyed() override;
   void RenderFrameDeleted(RenderFrameHost* render_frame_host) override;
   bool OnMessageReceived(const IPC::Message& message,
                          RenderFrameHost* render_frame_host) override;