Paint local background colors onto foreground layer.

third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-with-transparent-background-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
-          "name": "LayoutBlockFlow DIV class='scroller'",
+          "name": "LayoutBlockFlow DIV id='scroller'",
           "position": [8, 8],
-          "bounds": [300, 300],
+          "bounds": [220, 220],
           "shouldFlattenTransform": false,
           "drawsContent": true,
+          "backgroundColor": "#FFFFFF",
           "children": [
             {
               "name": "Scrolling Layer",
-              "bounds": [285, 285],
+              "position": [10, 10],
+              "bounds": [185, 185],
               "shouldFlattenTransform": false,
               "children": [
                 {
                   "name": "Scrolling Contents Layer",
-                  "bounds": [1000, 1000],
+                  "bounds": [185, 300],
                   "drawsContent": true
                 }
               ]
             },
             {
               "name": "Overflow Controls Host Layer",
-              "bounds": [300, 300],
+              "bounds": [220, 220],
               "children": [
                 {
                   "name": "Horizontal Scrollbar Layer",
-                  "position": [0, 285],
-                  "bounds": [285, 15]
+                  "position": [10, 195],
+                  "bounds": [185, 15]
                 },
                 {
                   "name": "Vertical Scrollbar Layer",
-                  "position": [285, 0],
-                  "bounds": [15, 285]
+                  "position": [195, 10],
+                  "bounds": [15, 185]
                 },
                 {
                   "name": "Scroll Corner Layer",
-                  "position": [285, 285],
+                  "position": [195, 195],
                   "bounds": [15, 15],
                   "drawsContent": true
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }