More natural way to serialize GPU tasks and tests.

This basically takes out the Windows-only hacks and promotes them to
cross-platform behavior driven by --gpu_threading.
    - When --gpu_threading is false (the default), this puts GPU tasks and tests
      together in the same GPU enclave.  They all run serially.
    - When --gpu_threading is true, both the tests and the tasks run totally
      independently, just like the thread-safe CPU-bound work.

BUG=skia:3255

Review URL: https://codereview.chromium.org/847273005
8 files changed