Teach WorkerThread about creation/destruction/management of child threads.

Bug: 829119

Change-Id: I8808486a29691cf0881864be000485322e5c2034
Reviewed-on: https://chromium-review.googlesource.com/996554
Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549664}
5 files changed