Introduce WorkerBackingThread

This change decouples thread-related part from WorkerThread. Now
multiple WorkerThread can be attached to one WorkerBackingThread
and one WorkerBackingThread is associated to one platform thread.

BUG=575532

Review URL: https://codereview.chromium.org/1733353004

Cr-Commit-Position: refs/heads/master@{#386948}
17 files changed