compositor-worker: Refactor CompositorWorkerManager

Much of the functionality in CompositorWorkerManager
was related to shared compositor thread state. This
patch moves that to CompositorWorkerSharedState, an
implementation detail of CompositorWorkerThread.

The shared state also now uses the real compositor
thread.

The existing compositor worker layout tests needed
to be moved to a virtual test suite since they now
depend on having a compositor thread.

BUG=430155

Review URL: https://codereview.chromium.org/1449953002

Cr-Commit-Position: refs/heads/master@{#362590}
30 files changed