Remove RenderLayer::enclosingScrollableArea

To avoid code duplication between RenderLayer and
RenderLayerScrollableArea, refactored the common
code into RenderObject::enclosingScrollableBox.

BUG=260899

Review URL: https://codereview.chromium.org/26654004

git-svn-id: svn://svn.chromium.org/blink/trunk@159287 bbb929c8-8fbe-4397-9dbb-9b2b20218538