Force flattening for elements with non-initial value of opacity

third_party/WebKit/LayoutTests/platform/win/compositing/geometry/preserve-3d-switching-expected.txt

 The green box appear angled out from the yellow box and embedded in it.
 
 transform: translateZ(-100px) rotateY(45deg);
 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
@@ skipping 14 matching lines @@
                 [1, 0, 0, 0],
                 [0, 1, 0, 0],
                 [0, 0, 1, -0.002],
                 [0, 0, 0, 1]
               ],
               "children": [
                 {
                   "name": "LayoutBlockFlow DIV id='parent'",
                   "position": [12, 12],
                   "bounds": [280, 280],
-                  "opacity": 0.800000011920929,
                   "contentsOpaque": true,
                   "shouldFlattenTransform": false,
                   "3dRenderingContext": 1,
                   "drawsContent": true,
                   "backgroundColor": "#FFFF00",
                   "transform": [
                     [0.766044443118978, -0.556670399226419, -0.32139380484327, 0],
                     [0, 0.5, -0.866025403784439, 0],
                     [0.642787609686539, 0.663413948168938, 0.383022221559489, 0],
                     [0, 0, 0, 1]
@@ skipping 19 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }