Fix the bug that negative outline-offset is covered up by composited

third_party/WebKit/LayoutTests/paint/invalidation/flexbox/scrollbars-changed-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow DIV id='scroller'",
@@ skipping 58 matching lines @@
           "object": "Vertical Scrollbar Layer",
           "rect": [0, 0, 15, 100],
           "reason": "full"
         },
         {
           "object": "Vertical Scrollbar Layer",
           "rect": [0, 0, 15, 100],
           "reason": "full"
         }
       ]
+    },
+    {
+      "name": "Decoration Layer",
+      "bounds": [200, 100],
+      "drawsContent": true
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow DIV id='dynamic' class='content'",
       "reason": "forced by layout"
     },
     {
       "object": "RootInlineBox",
       "reason": "forced by layout"
     },
     {
       "object": "LayoutText #text",
       "reason": "full"
     },
     {
       "object": "InlineTextBox 'z'",
       "reason": "full"
     }
   ]
 }