[GCM] Observe the event listener adding such that GCM can be started

BUG=349578
TEST=Manual test by following repro steps
R=fgorski@chromium.org

Review URL: https://codereview.chromium.org/189803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255729 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255729