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Currently you can request a reference to the MainThreadTaskQueue as well
as the underlying TaskQueue from the SchedulerHelper. In all cases, the
TaskQueue is only used to directly get the TaskRunner. We don't need to
return the TaskQueue here so this change returns a reference to the
TaskRunner instead.

This CL is a follow-up to this CL[1] and removes a spot where we need to
return a raw pointer to the underlying TaskQueue.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2453591

Bug: 1143007
Change-Id: I40e751f77447c54961c0102ddb596ddcfb035f69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519718Reviewed-by: Scott Haseley <shaseley@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824607}