A git cookie isn't enough, a user needs to associate their google
account with a gerrit one. Also clarify which users need to sign a CLA.

Fixes errors like:
remote: PERMISSION_DENIED: The caller does not have permission
remote: [type.googleapis.com/google.rpc.LocalizedMessage]
remote: locale: "en-US"
remote: message: "\'git push\' requires a Gerrit user account."
remote:
...
fatal: unable to access 'https://aomedia-review.googlesource.com/aom/':
The requested URL returned error: 403

Change-Id: I86ab89fcb5493b9300fdf5add9a2a12b577d8717