Remove usage of SequencedWorkerPoolOwner.

Replace usage with ScopedTaskEnvironment.

TBR=glider@chromium.org

Bug: 667892
Change-Id: I93d25b143a39f8966d425d98954bec28c710857a
Reviewed-on: https://chromium-review.googlesource.com/881532
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532260}
10 files changed