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It's an utility class that's currently only usable on full threads
and with a little effort can be made to only require a Sequence.

Bug: 978036
Change-Id: Iea6c57f81cb59000351534deae1dd445ad35dc03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675604
Auto-Submit: Tomasz Śniatowski <tsniatowski@vewd.com>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672499}