Migrate ContentHashFetcher off of BlockingPool

R=rdevlin.cronin@chromium.org

Bug: 667892
Change-Id: I21df3c5a2eac7d6a5d2b1a3bc0322d752baca286
Reviewed-on: https://chromium-review.googlesource.com/663767
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501607}
1 file changed