Add documentation for
the chrome.sockets API and manifest key.

BUG=293756

Review URL: https://codereview.chromium.org/94093013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243728 0039d316-1c4b-4281-b951-d872f2087c98
GCM Checkin implementation with unit tests.

BUG=284553

Review URL: https://codereview.chromium.org/98173009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243729 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243729