DevTools: fix co-existance of multiple DevTools extensions

Source/devtools/front_end/extensions/ExtensionAPI.js

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 871 matching lines @@
     unregisterHandler: function(command)
     {
         delete this._handlers[command];
     },
 
     /**
      * @return {string}
      */
     nextObjectId: function()
     {
-        return injectedScriptId + "_" + ++this._lastObjectId;
+        return injectedScriptId.toString() + "_" + ++this._lastObjectId;
     },
 
     _registerCallback: function(callback)
     {
         var id = ++this._lastRequestId;
         this._callbacks[id] = callback;
         return id;
     },
 
     _onCallback: function(request)
@@ skipping 101 matching lines @@
 {
     return "(function(injectedScriptId){ " +
         "var extensionServer;" +
         defineCommonExtensionSymbols.toString() + ";" +
         injectedExtensionAPI.toString() + ";" +
         buildPlatformExtensionAPI(extensionInfo, inspectedTabId) + ";" +
         "platformExtensionAPI(injectedExtensionAPI(injectedScriptId));" +
         "return {};" +
         "})";
 }