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This function creates a SequencedTaskRunner whose priority can be
updated at any time.

To update the priority from BEST_EFFORT to USER_VISIBLE/USER_BLOCKING,
a ThreadPolicy must be specified.

Bug: 889029
Change-Id: I4e6d119eda0a4a880922d49f97f84c30e8725516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532826
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664774}