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

third_party/WebKit/LayoutTests/compositing/scrollbars/nested-overlay-scrollbars-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow (positioned) DIV id='outer'",
@@ skipping 25 matching lines @@
       "position": [2, 2],
       "bounds": [200, 200],
       "shouldFlattenTransform": false
     },
     {
       "name": "Scrolling Contents Layer",
       "bounds": [5000, 9000],
       "drawsContent": true
     },
     {
+      "name": "Decoration Layer",
+      "bounds": [204, 204],
+      "drawsContent": true
+    },
+    {
       "name": "Squashing Containment Layer",
       "shouldFlattenTransform": false
     },
     {
       "name": "LayoutBlockFlow (positioned) DIV id='grey'",
       "position": [2, 502],
       "bounds": [100, 800],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#808080"
@@ skipping 26 matching lines @@
       "drawsContent": true
     },
     {
       "name": "Overflow Controls Host Layer",
       "bounds": [404, 404]
     },
     {
       "name": "Vertical Scrollbar Layer",
       "position": [395, 2],
       "bounds": [7, 400]
+    },
+    {
+      "name": "Decoration Layer",
+      "bounds": [404, 404],
+      "drawsContent": true
     }
   ]
 }