GitLab

Registering a KeepAlive during shtudown is forbidden and hits
a CHECK, so we should avoid starting some operations that do
it.

This CL adds checks to avoid such situations:
- push notifications received from GCM while shutting down
will be dropped early instead of causing a crash and being
dropped anyway.
- Extension functions using chrome_details validate before
running that the browser process is not shutting down.

BUG=625646

Review-Url: https://codereview.chromium.org/2118473002
Cr-Commit-Position: refs/heads/master@{#406001}