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Previously, the priority of the background worker was set via an
asynchronous callback. With SimpleThread::handle() which was added
in https://crrev.com/c/2578172, this can be simplified.

Change-Id: Ia50abab5647ef13412488e40f1589ec9d263dc86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578677
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835655}