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The event is useful in tracing. Now that we're looking at jank using
sampling profiler, we need similar annotation for stack samples.

Change-Id: I6987dcb7da856f4a2099479d16c3ca37fd9e113a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144775Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759733}