Unprefix usage of -webkit-transform in tests

LayoutTests/compositing/geometry/preserve-3d-switching-expected.txt

 The green box appear angled out from the yellow box and embedded in it.
 
--webkit-transform: translateZ(-100px) rotateY(45deg);
+transform: translateZ(-100px) rotateY(45deg);
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [108, 73],
@@ skipping 44 matching lines @@
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }