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Because of the unusual way that the TouchEmulator creates pinch events,
it's possible for a GestureFlingStart (created by the GestureRecognizer)
to terminate a pinch sequence. In that event a synthetic
GestureScrollEnd is inserted.

Prior to this CL, that GestureScrollEnd had a zero value
for unique_touch_event_id, which in turn caused problems when it was
sent to DispatchTouchscreenGestureEvent, along with a null target.

Bug: 900496
Change-Id: I290757980def3f55a0111a2e31f8424ce06e96f1
Reviewed-on: https://chromium-review.googlesource.com/c/1312619
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605432}