GitLab

Removes GetCompositorMessageLoop from the ContextFactory interface,
and drops the compositor_thread_ members from the ContextFactory
implementations.

There is no compositor thread in the ui compositor so these are now
excess complication.

R=sievers
BUG=329552

Review URL: https://codereview.chromium.org/943763004

Cr-Commit-Position: refs/heads/master@{#317622}