Reland "Change autoscroll latching to top-most delta-consumable scroller"

This is a reland of 07b882d4

The original CL was reverted due to a few layout tests failing in ASAN.
The cause of these failures was that vertical_autoscroll_layout_box_
and horizontal_autoscroll_layout_box were not being cleared in
StopMiddleClickAutocroll. This resulted in heap-use-after-free errors.

In order to fix this issue, the two pointers are cleared in
StopMiddleClickAutoscroll. They are also cleared if necessary in
StopAutoscrollIfNeeded. In the latter, middle click autoscroll would be
stopped if both layout boxes are to be cleared.

Original change's description:
> 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}

Bug: 1107648
Change-Id: Idf3c2253a25d2cfbe12a8ffe30bbf697c636d222
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582733


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@{#842593}