compositor-worker: Keep worker backing thread alive for the lifetime of the compositor thread.

This change is to support creating and holding onto oilpan objects on the compositor thread
in https://codereview.chromium.org/1895873006

BUG=430155
TEST=CompositorWorkerThreadTest.*

Review-Url: https://codereview.chromium.org/1955693003
Cr-Commit-Position: refs/heads/master@{#396732}
5 files changed