Update MediaPlayerRendererWebContentsObserver comment

Updating the comment per miu@'s comments on crrev.com/c/713406

Bug: 765337
Change-Id: I80a1ebaabfa70479f724cb2b86697fc3231db61e
Reviewed-on: https://chromium-review.googlesource.com/726385Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510236}

content/browser/media/android/media_player_renderer_web_contents_observer.h

@@ -15,7 +15,9 @@ class MediaPlayerRenderer;
 
 // This class propagates WebContents muting updates to MediaPlayerRenderers.
 // This allows us to avoid adding N WebContentsObservers for N
-// MediaPlayerRenderers on a page.
+// MediaPlayerRenderers on a page. Essentially, this is a call-stack filter to
+// prevent uninteresting observer methods from calling into the
+// MediaPlayerRenderers.
 class MediaPlayerRendererWebContentsObserver
     : public WebContentsObserver,
       public WebContentsUserData<MediaPlayerRendererWebContentsObserver> {