Avoid double scrollbars when the HTML element is not the render view

The code in RenderLayerScrollableArea for updating after a style change
was slightly wrong. It attempted to avoid creating extra scrollbars for
the viewport but doesn't quite handle the html/body duality correctly.

The code is convoluted, but the scrollbars of the FrameView are
determined by the scrollbar modes (in addition to the document size,
obviously), and these modes are computed in
FrameView::calculateScrollbarModesForLayoutAndSetViewportRenderer
based on the "viewport defining element." The code in
RenderLayerScrollableArea was a crude approximation of viewport
defining element. I've changed the code to make this more explicit.

BUG=374156

Review URL: https://codereview.chromium.org/451063002

git-svn-id: svn://svn.chromium.org/blink/trunk@180600 bbb929c8-8fbe-4397-9dbb-9b2b20218538