Remove assumption that null containerForRepaint assumes RenderView

Previously, when containerForRepaint was null, this would mean that
either the renderer wasn't rooted, or its container for repaint was
a non-composited RenderView. This patch changes containerForRepaint
to only return null when the renderer isn't rooted, and to actually
return the RenderView when that's the container. This allows
many of the callsites to be simplified to not check for null, and
removes the need of callers to understand the general assumption
that null means RenderView.

BUG=363699

Review URL: https://codereview.chromium.org/247803003

git-svn-id: svn://svn.chromium.org/blink/trunk@172531 bbb929c8-8fbe-4397-9dbb-9b2b20218538