Remove BlockingPool usage from ProfileImpl

This CL moves away from the JsonPrefStore::GetTaskRunnerForFile()
function, which was requiring a blocking pool.
Instead the task runner management is done by hand in the class.

Bug: 667892
Change-Id: Ie2485a70d79f75b69c3bf4543d48815bc1d78904
Reviewed-on: https://chromium-review.googlesource.com/575229
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489906}
2 files changed