Move GCMProfileService away from listening for Signin notifications.

As part of the effort to eliminate Signin-related notifications, move
GCMProfileService to observing the SigninManager.

BUG=345619
R=zea@chromium.org

Review URL: https://codereview.chromium.org/189063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256207 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256207
Updated //components/README to account for layered components.

R=joi@chromium.org

Review URL: https://codereview.chromium.org/194673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256208 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256208