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Before this change TouchEventFilter filtering out touch events that
happen outside of the allowed area would not only filter out taps, but
also scrolls.

This was inconvenient, as users would need to know that they need to
scroll starting from the highlighted, touchable are. Users could also
fling the highlighted area completely out of view, which left them
unable to do anything.

With this change, TouchEventFilter does the scrolling when a touch event
hits a non-allowed area. This requires dealing with two kinds of
scrolling. Once started, scrolling is allowed to continue until the end,
even if the touch event leaves the touchable or non-touchable area.

This change also agressivel triggers update of the touchable area while
scrolling. With browser-controlled scrolling, this corrects any drift
that result from the UI and the page having different scrolling speeds.

Bug: 806868
Change-Id: I85e3fd1339fadf9efe43b793c62ab6ce81c9247b
Reviewed-on: https://chromium-review.googlesource.com/c/1335928
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608318}