Fix border painting on self-painting layers.

Commit efc3afd428bd85fd3d12e0dc941b5eb7248ca30b broke it because
we'd computed an empty foreground layer. Now that background
painting is part of the foreground phase, we need to use the
background rect for clipping. As best I can tell, the
background rect is always >= the size of the foreground
rect, so it should be safe to use.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/876243002

examples/flights-app/app-toast.sky

@@ -16,6 +16,7 @@
       display: flex;
       align-items: center;
       font-size: 0.8em;
+      overflow: hidden;
     }
 
     .header {