Enable accessible name of a control to include multiple <label> elements.

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

 {
   "id": "<int>",
   "result": {
     "accessibilityNode": {
       "nodeId": "<string>",
       "ignored": false,
       "role": {
         "type": "internalRole",
         "value": "Div"
       },
@@ skipping 191 matching lines @@
             "type": "attribute",
             "attribute": "aria-label",
             "superseded": true
           },
           {
             "type": "relatedElement",
             "superseded": true,
             "nativeSource": "label"
           },
           {
             "type": "contents",

[aboxhall, 2016/06/03 21:19:44]

Hmmm something weird has happened here. Is this a consequence of the change to
recursiveTextAlternative?

Yeah, looks like it, because recursiveTextAlternative is called at
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/access...

Ugh. Seems like we need to make a copy of |visited|.

-            "value": {
-              "type": "computedString",
-              "value": "Contents"
-            },
             "superseded": true
           },
           {
             "type": "attribute",
             "attribute": "title",
             "superseded": true
           }
         ]
       },
       "properties": [
@@ skipping 64 matching lines @@
             "attribute": "aria-label",
             "superseded": true
           },
           {
             "type": "relatedElement",
             "superseded": true,
             "nativeSource": "label"
           },
           {
             "type": "contents",
-            "value": {
-              "type": "computedString",
-              "value": "Contents"
-            },
             "superseded": true
           },
           {
             "type": "attribute",
             "attribute": "title",
             "superseded": true
           }
         ]
       },
       "properties": [
@@ skipping 1016 matching lines @@
                 "nodeResult": "ul#list1"
               }
             ]
           }
         }
       ]
     }
   }
 }