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The sequence of events causing the flake is as follows:

1) when flushing the touch event queue, we ack a touch event and
   generate a scroll update gesture event.
2) Before dispatching the GSU, we prepend a touch event
   (kTouchScrollStarted) which is non-blocking and gets acked
   immediately, causing us to run AckCompletedEvents.
3) AckCompletedEvents acks the remaining touch events in the queue, and
   in this test, acks a touch cancel event which generates and
   dispatches a scroll end gesture event.
4) The initial scroll update event is actually dispatched after the
   scroll end event (which is an invalid ordering).

To fix this, we prevent AckCompletedEvents from being called while
we're flushing the queue to prevent it from affecting gesture event
ordering.

Bug: 1010675
Change-Id: I00d102ccde0b4bd458d86737a660f5fb777a3c6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849430Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705156}