Fix nested border radius with composited child.

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

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