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ProfileSyncService mostly creates the task runner and passes it to the
engine, so this CL moves the creation to the engine. The one use of
the runner by ProfileSyncService is replaced with posting a task to the
thread pool.

Bug: 1123881
Change-Id: Ifef2806b527424b30e2a0d6ef07cfca026ee07b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461325Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#816106}