Expose whether a ScrollUpdate sequence has been partially prevented

This bit is useful for better understanding how the touch sequence that
gave rise to a given scroll sequence was handled by the page. In
particular, this affords the gesture consumer an opportunity to limit
itself if part of the gesture was consumed by the page's touch handler,
e.g., disabling a pull-to-refresh styled overscroll effect if the
initial motion was preventDefault'ed,
https://codereview.chromium.org/679493002.

Chromium-side wiring for this data will be added in
https://codereview.chromium.org/712133003.

BUG=428429

Review URL: https://codereview.chromium.org/690173002

git-svn-id: svn://svn.chromium.org/blink/trunk@185068 bbb929c8-8fbe-4397-9dbb-9b2b20218538