Don't squash layers that have a clip or overflow clip.

Don't squash layers that have a clip or overflow clip,
because they may use a composited graphics layer to
clip descendant layers.

BUG=366101

Review URL: https://codereview.chromium.org/260113004

git-svn-id: svn://svn.chromium.org/blink/trunk@172845 bbb929c8-8fbe-4397-9dbb-9b2b20218538