RenderIFrames should not force a self painting layer

It turns out that this causes paint inversions and clipping issues. We
don't want to make iframes OverflowClipLayers (because they have other
semantics), so I've introduced a new layer type here.

R=jchaffraix@chromium.org
BUG=342478

Review URL: https://codereview.chromium.org/180373002

git-svn-id: svn://svn.chromium.org/blink/trunk@168086 bbb929c8-8fbe-4397-9dbb-9b2b20218538