[GCM] Support registration before unregistration finishes

[GCM] Support registration before unregistration finishes

This is to solve the problem that the app could be uninstalled and
then reinstalled immediately. We do not want the app to encounter
ASYNC_OPERATION_PENDING error. We now wait till the unregistration
completes and then triggers the pending registration.

BUG=371880
TEST=new test added

Committed: https://crrev.com/f17b85ad7985ebf3685bfc9ac23895f29247f474
Cr-Commit-Position: refs/heads/master@{#301939}

[jianli, 2014-10-28 22:16:27]

jianli@chromium.org changed reviewers:
+ fgorski@chromium.org

[fgorski, 2014-10-29 20:44:40]

lgtm. Thanks for adding comments.

[jianli, 2014-10-29 20:46:52]

The CQ bit was checked by jianli@chromium.org

[commit-bot: I haz the power, 2014-10-29 20:49:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/683913005/20001

[commit-bot: I haz the power, 2014-10-29 21:42:51]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-10-29 21:43:16]

Patchset 2 (id:??) landed as
https://crrev.com/f17b85ad7985ebf3685bfc9ac23895f29247f474
Cr-Commit-Position: refs/heads/master@{#301939}