[ChromeDriver] Add support for attaching multiple DevToolsEventListeners to a DevToolsClient.

chrome/test/chromedriver/devtools_client.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_TEST_CHROMEDRIVER_DEVTOOLS_CLIENT_H_
 #define CHROME_TEST_CHROMEDRIVER_DEVTOOLS_CLIENT_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 class DictionaryValue;
 }
 
+class DevToolsEventListener;
 class Status;
 
 // A DevTools client of a single DevTools debugger.
 class DevToolsClient {
  public:
   virtual ~DevToolsClient() {}
 
   virtual Status SendCommand(const std::string& method,
                              const base::DictionaryValue& params) = 0;
   virtual Status SendCommandAndGetResult(
       const std::string& method,
       const base::DictionaryValue& params,
       scoped_ptr<base::DictionaryValue>* result) = 0;
+
+  virtual void AddListener(DevToolsEventListener* listener) = 0;
 };
 
 #endif  // CHROME_TEST_CHROMEDRIVER_DEVTOOLS_CLIENT_H_