Fix paint invalidation when composited layers change from paints-into-ancestor to self-painting.

LayoutTests/compositing/squashing/invalidate-when-leaving-squashed-layer-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
+      "repaintRects": [
+        [0, 0, 200, 200]
+      ],
       "children": [
         {
           "children": [
             {
               "position": [8, 8],
               "bounds": [200, 200],
               "contentsOpaque": true,
               "drawsContent": true,
               "backgroundColor": "#D3D3D3"
             },
@@ skipping 16 matching lines @@
           "repaintRects": [
             [0, 0, 200, 200],
             [0, 0, 200, 200]
           ]
         }
       ]
     }
   ]
 }