remove rollback to xmpp when GCM disabled

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
Committed: https://crrev.com/bbe31010f9cd16c5c8dc659ce7d9598f0906b7e5
Cr-Commit-Position: refs/heads/master@{#330564}

[Gang Wu, 2015-05-13 00:07:43]

gangwu@chromium.org changed reviewers:
+ pavely@chromium.org

[Gang Wu, 2015-05-13 18:16:08]

please take a look.

[pavely, 2015-05-13 21:19:06]

lgtm

[Gang Wu, 2015-05-19 17:26:02]

The CQ bit was checked by gangwu@chromium.org

[commit-bot: I haz the power, 2015-05-19 17:26:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1127393005/10002

[commit-bot: I haz the power, 2015-05-19 18:22:21]

Committed patchset #2 (id:10002)

[commit-bot: I haz the power, 2015-05-19 18:23:56]

Patchset 2 (id:??) landed as
https://crrev.com/bbe31010f9cd16c5c8dc659ce7d9598f0906b7e5
Cr-Commit-Position: refs/heads/master@{#330564}