Switch to using a communication token for the RSS extension iframe. 

BUG=Internal (8503404)

Review URL: https://codereview.chromium.org/108783006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243285 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243285