Split computeScrollDimensions() out of RenderLayer

This change also moves 2 members of RenderLayer to
RenderLayerScrollableArea and a lot of related code to the
new class. In particular, the after-layout and after-style-change
notification had to be added.

BUG=260899

Review URL: https://chromiumcodereview.appspot.com/22893055

git-svn-id: svn://svn.chromium.org/blink/trunk@157192 bbb929c8-8fbe-4397-9dbb-9b2b20218538