Don't have AudioFocusWebContentsObserver declare itself as a friend

This is redundant since a class already has access to its own members.

Bug: None
Change-Id: I567ae3d63d3fc81b06a3dc73723c7df6c88d6aa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551251
Commit-Queue: Ben Wells <benwells@chromium.org>
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830109}

chrome/browser/apps/platform_apps/audio_focus_web_contents_observer.h

@@ -22,7 +22,6 @@ class AudioFocusWebContentsObserver
 
  private:
   friend class content::WebContentsUserData<AudioFocusWebContentsObserver>;
-  friend class AudioFocusWebContentsObserver;
   friend class AudioFocusWebContentsObserverBrowserTest;
 
   explicit AudioFocusWebContentsObserver(content::WebContents*);