Removes BlockingPool usage from chromecast::PrefServiceHelper.

Instead of using JsonPrefStore::GetTaskRunnerForFile, which requires a
blocking pool, we now instantiate JsonPrefStore directly which creates
an internal default task runner.

Bug: 667892
Change-Id: I3e0e037363efbc3fa806f5a7cbec0d4c7ec3a26b
Reviewed-on: https://chromium-review.googlesource.com/590482
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490236}
1 file changed