cc: Threaded GPU - fix thread binding.

cc: Threaded GPU - fix thread binding.

In theaded GPU raster mode, the compositor & worker threads can
take control of the worker_context_provider while holding the
context lock.

Currently if GpuTTWP binds to the thread and the compositor
needs to take ownership under lock, then the context's thread
checker may DCHECK().  Similarly in the reverse.

This change fixes binding of the context's thread checker,
allowing binding to the worker thread under lock, and detaching
before releasing the lock, so the compositor may bind again.

TBR=vmpstr@chromium.org
TBR=jbauman@chromium.org
BUG=454500

Review URL: https://codereview.chromium.org/925183002

Cr-Commit-Position: refs/heads/master@{#316498}