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Fix URLFetcherTest.SequencedTaskTest to run things on the Sequence.

This test previously used the WaitingURLFetcherDelegate helper's APIs
to run a RunLoop on the main thread, and wait for it to be quit by a
fetch completing on the Sequence. However, this violated the constraint
that the RunLoop may only be created, run and destroyed from the same
thread.

Bug: 729716
Change-Id: I4b699d17da7888527ef8d22f8e73a7f91236ef23
Reviewed-on: https://chromium-review.googlesource.com/1056265
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558344}