Enable cpplint and fix cpplint errors in webrtc/api

webrtc/api/mediastreamtrackproxy.h

 /*
  *  Copyright 2011 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 // This file includes proxy classes for tracks. The purpose is
 // to make sure tracks are only accessed from the signaling thread.
 
 #ifndef WEBRTC_API_MEDIASTREAMTRACKPROXY_H_
 #define WEBRTC_API_MEDIASTREAMTRACKPROXY_H_
 
+#include <string>
+
 #include "webrtc/api/mediastreaminterface.h"
 #include "webrtc/api/proxy.h"
 
 namespace webrtc {
 
 BEGIN_SIGNALING_PROXY_MAP(AudioTrack)
   PROXY_SIGNALING_THREAD_DESTRUCTOR()
   PROXY_CONSTMETHOD0(std::string, kind)
   PROXY_CONSTMETHOD0(std::string, id)
   PROXY_CONSTMETHOD0(TrackState, state)
@@ skipping 24 matching lines @@
   PROXY_WORKER_METHOD1(void, RemoveSink, rtc::VideoSinkInterface<VideoFrame>*)
   PROXY_CONSTMETHOD0(VideoTrackSourceInterface*, GetSource)
 
   PROXY_METHOD1(void, RegisterObserver, ObserverInterface*)
   PROXY_METHOD1(void, UnregisterObserver, ObserverInterface*)
 END_PROXY_MAP()
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_MEDIASTREAMTRACKPROXY_H_