Compositor threaded scrollbar track clicks.

This CL implements handling mousedown on the scrollbar track (i.e the
part between the thumb and the arrows). The way this works is, when
you click on the track, the forward and back track rects and calculated
based on the back, forward and thumb rects. If the hit tested point is
contained in this rect, the ScrollbarController determines the scroll
delta based on the viewport_length. This delta is then added to a
synthetic GSU that is handled on the compositor thread.

Bug: 954238
Change-Id: I9c9769f07bdaf3e4d628d2175b7e07ae46228a94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615909
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661930}
10 files changed