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(this doesn't use the TaskScheduler API directly because that's the
default behavior on JsonPrefStore now when not provided an explicit
SequencedTaskRunner at construction :))

R=rohitrao@chromium.org

Bug: 667892
Change-Id: Ifa0549e4ddf65ad8606451a47724234981c35467
Reviewed-on: https://chromium-review.googlesource.com/590573Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490766}