Compositor threaded scrollbar scrolling - Implement autoscrolling

This CL implements mousedown autoscroll for scrollbar arrows using a
DocumentTimeline animation. The way this works is, when you press and
hold a auto-scrollable ScrollbarPart, a callback to create a scroll
animation is posted. This callback will execute if the ScrollbarPart
is held beyond 250ms. On mouseup, the animation is aborted hence
stopping the autoscroll. The autoscroll animation is constant and
perpetual until aborted.

Bug: 954011
Change-Id: I67c7c2c91cc725966f06d7a7633d61d48ebb1af2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600870
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674345}