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This is ground work for having ProfileSyncService's backend task runner use the
ThreadPool.

This class in particular affects LocalSynctTest.Shouldstart when work is done
fully on the Threadpool.

Bug: 1014464
Change-Id: Id91c6b1682648a6fdb38efef5145ab4d6af0b2e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865427
Commit-Queue: Oliver Li <olivierli@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707363}