Prevent squashing a layer into its stacking ancestor

Squashing a layer into a CLM owned by its stacking ancestor changes
paint order, since it causes the layer to paint after stacking
siblings that come after it in paint order but don't get squashed.

Review URL: https://codereview.chromium.org/288313004

git-svn-id: svn://svn.chromium.org/blink/trunk@174227 bbb929c8-8fbe-4397-9dbb-9b2b20218538