Invalidate scrolling contents layer background when child invalidated.

third_party/WebKit/LayoutTests/paint/invalidation/selection-within-opaque-composited-scroller-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='scroller'",
           "position": [8, 8],
           "bounds": [200, 200],
-          "contentsOpaque": true,
           "shouldFlattenTransform": false,
           "drawsContent": true,
           "backgroundColor": "#D3D3D3",
           "children": [
             {
               "name": "Scrolling Layer",
               "bounds": [185, 185],
               "shouldFlattenTransform": false,
               "children": [
                 {
                   "name": "Scrolling Contents Layer",
                   "bounds": [200, 1620],
+                  "contentsOpaque": true,
                   "drawsContent": true,
                   "paintInvalidations": [
                     {
                       "object": "LayoutText #text",
                       "rect": [0, 610, 21, 19],
                       "reason": "selection"
                     }
                   ]
                 }
               ]
@@ skipping 36 matching lines @@
     },
     {
       "object": "LayoutBlockFlow DIV id='scroller'",
       "reason": "selection"
     },
     {
       "object": "LayoutBlockFlow DIV id='target'",
       "reason": "selection"
     },
     {
+      "object": "Scrolling Contents Layer",
+      "reason": "selection"
+    },
+    {
       "object": "LayoutText #text",
       "reason": "selection"
     },
     {
       "object": "InlineTextBox 'test'",
       "reason": "selection"
     }
   ]
 }