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This is a precursor to
https://chromium-review.googlesource.com/c/chromium/src/+/1761362
which is making destination a mandatory trait.

@OWNERS: Please verify that these traits indeed intended to post
to the ThreadPool.

Bug: 968047
Change-Id: Ia7c9bb718325e5fac727421aa62b6e6034987d6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761363
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689077}