Fix a crash in VRBrowserRendererThreadWin

VRBrowserRendererThreadWin behaved as though it lived on a separate
thread, posting tasks to do work instead of doing the work
synchronously.  If it was destroyed, those posted tasks would still run,
potentially using VRBrowserRendererThreadWin or its members after
destruction.

The fix is to stop pretending it is a thread, avoiding the posted tasks.

BUG=921739

Change-Id: I147cccf9487386b95c25401d444294321fa925af
Reviewed-on: https://chromium-review.googlesource.com/c/1413413
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623303}
3 files changed