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Pinch zoom was incorrectly not being preventDefault'd sometimes, in two
different scenarios: either the transition from scroll->pinch, or from
fling->pinch. In both cases, events were being queued in the
TouchpadPinchEventQueue, then TouchpadPinchEventQueue::ProcessMouseWheelAck
was receiving ACKs, which TPEQ assumed were the ACKs for the events that it
had already queued. While this is often true, it was not always the case.
Sometimes, the event ACKs from the scroll or fling would arrive after the
pinch events had been queued already, in which case the ACKs would be
processed as if they were for pinch events, when they weren't. This
resulted in the pinch zoom not always being preventDefault'd.

This CL fixes this by checking the event that was being ACK'd vs the event
that TPEQ expects an ACK for, and if they don't match then it early outs
and ignores the ACK.

Bug: 908758
Change-Id: Ifbad8e3ae46947c75b4680dc516398b69cf2cc89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725349
Commit-Queue: Mario Bianucci <mabian@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688161}