Deduplicate part of RenderLayerScrollableArea/ScrollableArea.

ScrollableArea only has one subclass now. Make all it's virtual
methods be pure virtual and delete any methods that
RenderLayerScrollableArea overrides.

Eventually we should probably merge the two classes into one,
but ScrollableArea is in platform and RenderLayerScrollableArea
is in core/rendering, so it's non-trivial figuring out what the
end result should look like. Hopefully it will become more clear
as we delete more of the scrolling code.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/711483003