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The touch action filter currently relies on checking the
GestureScrollBegin event for whether we need to filter or not. It
assumes a pinch-zoom will cause a GSB with a pointer_count == 2 and so
pinch-zoom is filtered in that way, since we always bracket GesturePinch
events between a GSB and a GestureScrollEnd.

However, this breaks down for double-tap-drag zoom (i.e. double tap but
hold and drag the second tap to zoom). In this case, the pointer_count
is 1 so we don't filter it.

In this CL, we remove the bracketing of pinch gestures in the
double-tap-drag zoom case and allow activating the filter at
GesturePinchBegin. We must now track in the TouchActionFilter whether
we've seen a GestureScrollBegin so that we know whether to apply and
remove the filtering at GesturePinchBegin/GesturePinchEnd.

Bug: 865090
Change-Id: I25e426f6f48dbf0ebcecfbddee5e2d8cbcc40504
Reviewed-on: https://chromium-review.googlesource.com/c/1279236
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600937}