GitLab

Creates a mode that allows scrolling without a scrollbar in either
direction as well as the ability to treat scroll-specific events (e.g.
mousewheel events) as horizontal scroll inputs.

Changes:
* Adds a third scrollbar state: visible but not enabled
* Allows enabled/disabled/invisible state for vertical scrollbar as well
as horizontal (previously only horizontal scrollbar could be hidden/
disabled).
* Adds the option to treat vertical scroll events as horizontal, so that
for example a mousewheel or vertical scroll gesture can allow a
horizontal-scroll-only pane to scroll.

Bug: 1122214
Change-Id: I7f01b26b33ede76e1903ee37a493a2818b7b1103
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2433254
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813839}