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. Add a 'NeedsBeginFrames' async-event to show the duration of a
  client's request for begin-frames.
. Add a flow-event for 'did not produce' frames.
. Add a trace-event for when a client is throttled (and include
  the reason for throttling).

BUG=none

Change-Id: I65e601b54a683bef54142b98f6faf037cc22308e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125618
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754980}