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This allows selective activation of hang watching on the different
supported threads.

Bug: 1034046
Change-Id: Iac18db82d162cc88b4ca78134b74cfec96340e33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288196Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792386}