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Unthrottling clients as soon as they acked any begin frame was not
effective in preventing a huge backlog of OnBeginFrame() IPCs from
building up. Instead of allowing any ack to unthrottle, instead count
the number of sent BeginFrameArgs and received BeginFrameAcks.

To avoid problems where clients don't properly ack all OnBeginFrame()
messages but are still responsive, if the client acks the latest
BeginFrameArgs then we treat it like they have acked all outstanding
begin frames.

Bug: 1011441
Change-Id: I9d0b73a189945d03c4e0590b1acea52df020ee03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947186
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721467}