Add browser tests for extensions tab API.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "Port",
         "type": "object",
         "description": "An object which allows two way communication with other pages.",
         "properties": {
           "name": {"type": "string"},
@@ skipping 928 matching lines @@
           {"type": "string", "name": "message"}
         ]
       },
       {
         "name": "notifyPass",
         "type": "function",
         "description": "Notify the browser process that test code running in the extension passed.  This is only used for internal unit testing.",
         "parameters": [
           {"type": "string", "name": "message", "optional": true}
         ]
+      },
+      {
+        "name": "log",
+        "type": "function",
+        "description": "Logs a message during internal unit testing.",
+        "parameters": [
+          {"type": "string", "name": "message"}
+        ]
       }
     ],
     "events": []
   }
 ]