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This reverts commit 153b3d46.

Reason for revert: https://crbug.com/1158103

Original change's description:
> [cc] Simplify lowering the priority of the background worker.
>
> 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}

TBR=fdoray@chromium.org,haraken@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com
Bug: 1158103

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ia4ad30cf20d2ccd406cfff6a75317183a889a0f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587801Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836424}