GitLab

We need to start GCMChannelStatusSyncer when GCM is disabled in order
to poll the server to find out when it is reenabled.

Also if server returns empty response, we should not treat it as error
and trigger the backoff logic.

BUG=423415
TEST=new tests added

Committed: https://crrev.com/9e9dd3b7798b3500d70941af04b1325ef9a0b544
Cr-Commit-Position: refs/heads/master@{#299518}

R=fgorski@chromium.org

Review URL: https://codereview.chromium.org/653843003

Cr-Commit-Position: refs/heads/master@{#299854}