Non-composited scrolling div should invalidate when it has descendant paints to its backing

This CL fixes a bug that non-composited scrolling div forgets to invalidate
because the div believed all its descandants are composited, when actually
some descendants paints to ancestor backing although composited.

BUG=401047

Review URL: https://codereview.chromium.org/460883002

git-svn-id: svn://svn.chromium.org/blink/trunk@180309 bbb929c8-8fbe-4397-9dbb-9b2b20218538