Don't squash into RenderPart subclasses. Don't squash into video elements.

LayoutTests/compositing/squashing/dont-squash-into-iframes-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
-          "position": [8, 8],
-          "bounds": [200, 200],
-          "contentsOpaque": true,
-          "drawsContent": true,
-          "backgroundColor": "#D3D3D3"
-        },
-        {
           "bounds": [104, 104],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#ADD8E6"
+        },
+        {
+          "position": [50, 50],
+          "bounds": [200, 200],
+          "contentsOpaque": true,
+          "drawsContent": true,
+          "backgroundColor": "#D3D3D3"
         }
       ]
     }
   ]
 }