Make BrowserMediaPlayerManager a WebContentsObserver instead of a RenderViewHostObserver so that it can filter IPCs.

This is part of the work in removing RenderViewHostObserver.

BUG=306569
R=qinmin@chromium.org

Review URL: https://codereview.chromium.org/36323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230789 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230789