Replace MessageLoop::current() with ThreadTaskRunnerHandle::Get() in GCM

Usages of MessageLoop::current() in GCM production code have been
replaced with ThreadTaskRunnerHandle::Get(). Usages of MessageLoop
in some test codes have also been replaced, but not all since we
don't have alternative to replace RunLoop.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/1232193002

Cr-Commit-Position: refs/heads/master@{#338401}
15 files changed