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Do not let the invalidations service to fall back to XMPP if GCM is disabled, since we already finished traansite from GCM to XMPP, and XMPP do not have prepare for such load spike.
Since Sync have backoff mechanism, so if GCM server some how disabled, sync will retry it later.

BUG=479300

Review URL: https://codereview.chromium.org/1127393005

Cr-Commit-Position: refs/heads/master@{#330564}