[GCM] Make sure delegate method is invoked when GCMClient gets ready in either case

Also rename GCMClient::Delegate::OnLoadingCompleted to OnGCMReady and
GCMClient::IsLoading to GCMClient::IsReady.

BUG=284553
TEST=existing tests

Review URL: https://codereview.chromium.org/149723006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248123 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248123