[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in signin

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I21e65426d9d25b9b9e654c6edf8c255ee17e7f34
Reviewed-on: https://chromium-review.googlesource.com/c/1353888
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#614657}
6 files changed