Prevent squashing into your own compositing container

If you squash on to your compositing container, you'll draw in the wrong order
because you'll draw after other RenderLayers that come after you in paint order
into your compositing container.

R=chrishtr@chromium.org
BUG=370862

Review URL: https://codereview.chromium.org/277763002

git-svn-id: svn://svn.chromium.org/blink/trunk@173709 bbb929c8-8fbe-4397-9dbb-9b2b20218538