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This CL creates a feature experiment that removes blocking worker pools
from the task scheduler. Tasks with MayBlock trait are instead posted to
the usual foreground/background pool, giving us better control over scheduling.

Note: ScopedBlockingCall is used to add extra workers to the pool when a task is
blocking. In this CL, we take advantage of this behavior to make sure enough
workers are running on the CPU.

Bug: 874080
Change-Id: I7889a941f82203388c259b0d96073eb5ddf3de69
Reviewed-on: https://chromium-review.googlesource.com/c/1249836
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600401}