GpuMain: Fix thread_runner() access DCHECK fail

Accessing base::Thread::task_runner() on an arbitrary thread is unsafe.
This causes a DCHECK to fail on some mus_ws_unittests. This CL saves
the task runner for the gpu thread and the compositor thread in member
variables to ensure safe access to the task runners.

BUG=none

Review-Url: https://codereview.chromium.org/2736693003
Cr-Commit-Position: refs/heads/master@{#454891}
2 files changed