ASSERT that squashing can't hit some repainting code paths

This CL builds upon Chris Harrelson's work in
https://codereview.chromium.org/224243003. Rather than fixing these code paths
to support squashing, this CL ASSERTs that we can never hit these code paths
when we paint into a grouped backing.

BUG=358261

Review URL: https://codereview.chromium.org/238023002

git-svn-id: svn://svn.chromium.org/blink/trunk@171544 bbb929c8-8fbe-4397-9dbb-9b2b20218538