Remove UserGesture on touch scrolls

The only type of touch which now takes a user gesture is touchend/pointerup
when it doesn't correspond to a touch scroll.  Conceptually scrolling
consume's the user's intent - no sensitive action should be triggered for
the same gesture that does a scroll.  For details see:
https://docs.google.com/document/d/1oF1T3O7_E4t1PYHV6gyCwHxOi3ystm0eSL5xZu7nvOg/edit

Intent to remove: https://groups.google.com/a/chromium.org/forum/#!searchin/blink-dev/gesture%7Csort:relevance/blink-dev/TO_x7FRkdmw/uCrOwufTCAAJ

BUG=611981

Review-Url: https://codereview.chromium.org/2414273003
Cr-Commit-Position: refs/heads/master@{#430330}
16 files changed