Implement aria-modal

third_party/WebKit/LayoutTests/inspector-protocol/accessibility/accessibility-ignoredNodesModal-expected.txt

 Hidden by modal dialog
 (Dialog itself is not hidden)
 Non-hidden element inside dialog
 {
   "nodeId": "<string>",
   "ignored": true,
   "ignoredReasons": [
     {
       "name": "activeModalDialog",
       "value": {
@@ skipping 30 matching lines @@
       {
         "type": "attribute",
         "attribute": "aria-label"
       },
       {
         "type": "attribute",
         "attribute": "title"
       }
     ]
   },
-  "properties": [],
+  "properties": [
+    {
+      "name": "modal",
+      "value": {
+        "type": "boolean",
+        "value": true
+      }
+    }
+  ],
   "domNode": "dialog"
 }
 {
   "nodeId": "<string>",
   "ignored": false,
   "role": {
     "type": "internalRole",
     "value": "Div"
   },
   "name": {
@@ skipping 11 matching lines @@
       {
         "type": "attribute",
         "attribute": "title"
       }
     ]
   },
   "properties": [],
   "domNode": "div"
 }