Remove RenderLayer::hasOverlayScrollbar

While touching the code, this change removed 2 confusing methods
includeVerticalScrollbarSize and includeHorizontalScrollbarSize.
Those were implictly checking for overflow: overlay scrollbars.

Also consolidated the logic in RenderBox::overflowClipRect
by checking hasOverflowClip() and calling the methods
directly RenderLayerScrollableArea.

BUG=260899

Review URL: https://codereview.chromium.org/27183012

git-svn-id: svn://svn.chromium.org/blink/trunk@159728 bbb929c8-8fbe-4397-9dbb-9b2b20218538