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This adds a new task queue API that returns a handle for the posted
task, and allows future tasks to take this handle as a parameter. This
creates a dependency between the two tasks, and the prerequisite task
will pass its return value as a parameter to the dependent task.

Also, fix a Persistent-rooted reference cycle that was causing
ThreadPools (and their associated threads) to leak.

Bug: 879306
Change-Id: I9af6b85ca2e56da464d4316b29bda53ba694e35a
Reviewed-on: https://chromium-review.googlesource.com/c/1242074
Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597386}