Remove scrollHeight / scrollWidth from RenderLayer

Converted most callers to use RenderBox to cut RenderLayer
dependencies. While at it, removed some bad dependencies
on RenderLayer inside RenderLayerScrollableArea.

The rebaselines come from RenderListBoxes as we now go
through RenderListBox::scrollHeight instead of
RenderLayer::scrollHeight. Either result is fine but
it makes some sense to match what JavaScript would get.

BUG=260899

Review URL: https://codereview.chromium.org/26520004

git-svn-id: svn://svn.chromium.org/blink/trunk@159679 bbb929c8-8fbe-4397-9dbb-9b2b20218538