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Add flow events to blink::AsyncTask to be able to see the connection
between the place requesting task execution and the task execution
itself for tasks like requestAnimationFrame.

This patch also adds TRACE_EVENT_{BEGIN,END}_WITH_FLOW{0,1,2}, which are
needed to implemented trace events for AsyncTask.

This patch also adds a helper GetGlobalUniqueFlowId(), which helps to
generate a unique flow id for tasks like these.

Note that a similar set of events exists in devtools categories, but
it uses different of set events, which are displayed in devtools UI, but
not in chrome://tracing.

R=eseckler@chromium.org,haraken@chromium.org
BUG=1142207

Change-Id: Id87bdb574a52752c4735a2a056d3d0a3d23ec342
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494877
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823762}