Repaint squashing content in cases where the entire frame is repainting.

In the process, inline RenderLayerRepainter::setBackingNeedsRepaint's one call site
and remove the method.

BUG=384520

Review URL: https://codereview.chromium.org/337793004

git-svn-id: svn://svn.chromium.org/blink/trunk@176150 bbb929c8-8fbe-4397-9dbb-9b2b20218538