Change autoscroll latching to top-most delta-consumable scroller

Users will now be able to use middle click autoscroll to scroll a
parent div if the inner-most scroller is unable to scroll in that
direction.

If there is no delta-consumable scroller, the top-most autoscrollable
scroller will be latched.

Bug: 1107648
Change-Id: Iccd4efec3b1ce5d09c701d3d46052176275dbc32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488042


Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Rahul Arakeri <arakeri@microsoft.com>
Commit-Queue: Sahir Vellani <sahir.vellani@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#835318}