Implement aria-placeholder

third_party/WebKit/LayoutTests/inspector-protocol/accessibility/accessibility-nameSources-labelledby-expected.txt

 {
   "nodeId": "<string>",
   "ignored": false,
   "role": {
     "type": "internalRole",
     "value": "Div"
   },
   "name": {
     "type": "computedString",
     "value": "",
@@ skipping 1196 matching lines @@
         "type": "relatedElement",
         "superseded": true,
         "nativeSource": "label"
       },
       {
         "type": "placeholder",
         "attribute": "placeholder",
         "superseded": true
       },
       {
+        "type": "placeholder",
+        "attribute": "aria-placeholder",
+        "superseded": true
+      },
+      {
         "type": "attribute",
         "attribute": "title",
         "superseded": true
       }
     ]
   },
   "properties": [
     {
       "name": "invalid",
       "value": {
@@ skipping 32 matching lines @@
             "text": "A B C",
             "nodeResult": "ul#list1"
           }
         ]
       }
     }
   ],
   "domNode": "input#input1"
 }