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This CL is moving the computation of engaged sites to the worker pool.

Move the worker model to a single static function to avoid any
incorrect fields accesses. The worker compute the sites and
pass it to the main thread.

The callbacks are queued and executed when at the completion of the
background computation.

Bug: 1157596
Change-Id: I4021d87fde51d673f78154b6cd5fc94fa71356d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627758Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843572}