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The task runner is used at UpdateInternalInstanceId and DeleteSoon, and
I think unthorttled task runner is not necessary here.

Note that the original task runner before kUnthrottled was
ThreadTaskRunnerHandle::Get().

We plan to eliminate kUnthrottled, kUnspecedTimer and kUnspecedLoading
and replace them with appropriate kInternal* task types.

Task type guideline: https://bit.ly/2vMAsQ4

Bug: 836410
Change-Id: I11679337908b6c64f3f6604639f2aed3b7c60ab0
Reviewed-on: https://chromium-review.googlesource.com/1055367Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559430}