chromeeos: Don't use GetBlockingPool for quirks code.

Update QuirksManager to use a base::TaskRunner from
base::CreateTaskRunnerWithTraits() instead of a
base::SequencedWorkerPool from
content::BrowserThread::GetBlockingPool(). As far as I can
tell, it was just treating the sequenced pool as a regular
task runner anyway.

BUG=667892

Change-Id: I2d198781bc27b17ad64b31b5f6c5b39bffee0406
Reviewed-on: https://chromium-review.googlesource.com/560897
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484613}
4 files changed