Paint local attachment backgrounds into composited scrolling contents layer

third_party/WebKit/LayoutTests/compositing/layer-creation/overlap-transformed-layer-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "name": "LayoutBlockFlow DIV id='container'",
           "position": [23, 8],
           "transformOrigin": [377, 0],
           "bounds": [256, 256],
           "contentsOpaque": true,
           "drawsContent": true,
           "transform": [
             [1, 0, 0, 0],
             [0, 1, 0, 0],
             [0, 0, 1, 0],
             [-10, 0, 0, 1]
           ],
           "children": [
             {
               "name": "LayoutBlockFlow (positioned) DIV id='composited'",
-              "position": [385, 0]
+              "position": [385, 0],
+              "contentsOpaque": true
             }
           ]
         },
         {
           "name": "LayoutBlockFlow (positioned) DIV id='green'",
           "bounds": [300, 300],
           "contentsOpaque": true,
           "drawsContent": true,
           "backgroundColor": "#008000"
         }
       ]
     }
   ]
 }