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

LayoutTests/platform/linux/compositing/video/video-poster-expected.txt

 A <video> should not have a layer until playback begins.
 This test can not be run manually, it requires testRunner.layerTreeAsText.
 
 Case: No src, no poster
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
@@ skipping 19 matching lines @@
 Case: Displaying movie
 {
   "bounds": [785, 813],
   "children": [
     {
       "bounds": [785, 813],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
+          "position": [8, 8],
+          "bounds": [352, 288]
+        },
+        {
           "children": [
             {
               "position": [8, 8],
-              "bounds": [352, 288]
+              "bounds": [352, 288],
+              "drawsContent": true
             },
             {
               "position": [8, 8],
-              "bounds": [352, 288],
+              "bounds": [352, 253],
               "drawsContent": true
             }
           ]
         }
       ]
     }
   ]
 }