Move part of the accelerated scrolling code

This change moves half of the accelerated scrolling functions
to RenderLayerScrollableArea. This removes all functions from
ScrollableArea on RenderLayer (though there are more to move).

While touching ScrollingCoordinatorChromiumTest, changed the
testing to be RenderBox based so as to minimize future changes.

BUG=260899

Review URL: https://codereview.chromium.org/40403003

git-svn-id: svn://svn.chromium.org/blink/trunk@160767 bbb929c8-8fbe-4397-9dbb-9b2b20218538