Adding WeakPtrFactory to GCMClientImpl, cleaning up .h a little bit

This patch addresses the comments from
https://codereview.chromium.org/130983003/

BUG=284553

Review URL: https://codereview.chromium.org/137263020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248257 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248257