sync: Fix crash on backend initialize failure

sync: Fix crash on backend initialize failure

Fixes a crash in ungraceful sync backend shutdown.  The problem was that
the sync backend host core might try to unregister itself as a listener
of some information generated in the ModelTypeRegistry, but the
ModelTypeRegistry did not exist if backend initialization failed.

The fix used here is to track whether or not we were registered for
these notifications in the first place, and only unregister if we
had previously registered successfully.  In the failure shutdown
case, the flag is unset, so we will not attempt to unregister.

BUG=376863
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=272697

[rlarocque, 2014-05-23 23:18:33]

Please review.

[maniscalco, 2014-05-23 23:38:35]

LGTM.  Let's get this landed.  We can work on cleaning up sync manager
initialization in the future.

[rlarocque, 2014-05-23 23:42:02]

The CQ bit was checked by rlarocque@chromium.org

[commit-bot: I haz the power, 2014-05-23 23:42:31]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rlarocque@chromium.org/294023017/1

[rlarocque, 2014-05-24 00:22:05]

The CQ bit was checked by rlarocque@chromium.org

[commit-bot: I haz the power, 2014-05-24 00:23:55]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rlarocque@chromium.org/294023017/1

[commit-bot: I haz the power, 2014-05-24 03:16:17]

Change committed as 272697