Fix a crash in the GPU watchdog destructor.

It might take long time to stop the watchdog thread and the GPU process
is terminated after the watchdog timeout is triggered. Disarm the
watchdog and invalidate the timeout task before stopping the watchdog
thread to avoid a crash.

Bug: 949839
Change-Id: Iccd61bf87c4dd5378af0670b66d031cf381ece56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1990130
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Maggie Chen <magchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729171}