chromeos: Drop GetBlockingPool from kiosk code.

Update kiosk classes and AutomaticRebootManager, along with
related tests, to call base::PostTask* instead of
content::BrowserThread::GetBlockingPool().

BUG=667892

Change-Id: Ie6db5d105ab872af5d6688a274c6b4106ddf1072
Reviewed-on: https://chromium-review.googlesource.com/562632
Reviewed-by: Satoru Takabayashi <satorux@google.com>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484842}
7 files changed