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This is a follow up to CL (https://chromium-review.googlesource.com/c/chromium/src/+/941264).
LazySingleThreadTaskRunner allows the created single-thread task runner be be
recycled in unittests.

Bug: 815225
Change-Id: I70402798883f3550aced26ef07d48d8e882967f4
Reviewed-on: https://chromium-review.googlesource.com/964782Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543822}