Wrapping message into ClientGatewayMessage is specific to PushClient

When cacheinvalidations library send  a message it should be treated
differently by PushClient channel vs. GCMNetworkChannel.
PushClientChannel should wrap message into ClientGatewayMessage while
GCMNetworkChannel should not.
Currently this functionality is in base class (SyncNetworkChannel).
Moving it to PushClientChannel along with corresponding unittests.

BUG=325020
R=rlarocque@chromium.org

Review URL: https://codereview.chromium.org/183893007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254284 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254284