Speculative fix for RenderLoop::ExitPresent crash

It looks as though (at least on Android), it's possible for ExitPresent
to be called in response to a StopRuntime call. The JavaHandlerThread
appears to teardown/destroy the task_runner before the StopRuntime call
thus resulting in a crash. It does look as though base::Thread does not
tear down the task_runner before this point, but StopRuntime is already
resilient to re-entrant calls, such that it doesn't seem worth investing
in a more complete fix to fully avoid that further re-entrant behavior.

Fixed: 1521499
Change-Id: Iea6dc2855d8a52cec31cca1a74f3182fbba79c1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5241898
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1252992}