[GCM] Fix mcs_probe credential handling

The mcs probe was no longer consuming credentials passed via command line
switch properly. This fixes it so that if credentials as passed, they're
used, otherwise if the credentials are restored they are used, and lastly
if no credentials are found a checkin is performed.

BUG=284553

Review URL: https://codereview.chromium.org/144383005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247248 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247248