[GCM] Remove user checkin for GCM.

Only one check-in is needed after we switched to one gcm connection per profile.

BUG=284553
TEST=tests updated
R=fgorski@chromium.org, zea@chromium.org

Review URL: https://codereview.chromium.org/139983005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250788 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250788