Move WorkerScheduler(Proxy) from scheduler/child/.

This patch moves files for WorkerScheduler and WorkerSchedulerProxy
from scheduler/child/ to the directories designated by the new
scheduler architecture plan. Out of those files, worker_thread.h is
used by Blink, so it is moved to scheduler/public/. Other files
go to scheduler/worker/.

Bug: 826203
Change-Id: I7c594979ceb25be8cca11b89f2cf2d3e05b1232b
Reviewed-on: https://chromium-review.googlesource.com/1082187
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564043}
16 files changed