Paint local attachment backgrounds into composited scrolling contents layer

third_party/WebKit/LayoutTests/fast/borders/overflow-hidden-border-radius-force-backing-store-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "name": "LayoutBlockFlow (positioned) DIV id='outer'",
           "position": [50, 50],
           "bounds": [300, 300],
           "drawsContent": true,
           "children": [
             {
               "name": "Child Containment Layer",
               "bounds": [300, 300],
               "children": [
                 {
                   "name": "Ancestor Clipping Layer",
                   "bounds": [285, 300],
                   "shouldFlattenTransform": false,
                   "children": [
                     {
                       "name": "LayoutBlockFlow DIV id='content'",
                       "bounds": [285, 1000],
+                      "contentsOpaque": true,
                       "drawsContent": true
                     }
                   ]
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }