Migrate iOS ApplicationContext to TaskScheduler.

(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/590573
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490766}
2 files changed