Rename and move WorkerTaskQueue.

The WorkerTaskQueue is actually for NonMainThreadScheduler, so it is
renamed to NonMainThreadTaskQueue. At the same time, the relevant files
are moved from scheduler/child/ to scheduler/worker/, which is the new
directory for worker-related files designated by the on-going scheduler
refactoring.

Bug: 826203
Change-Id: I3043ea9e182159978f94ff1bc8ef1af62a2029b0
Reviewed-on: https://chromium-review.googlesource.com/1082059
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564025}
18 files changed