Enable cpplint and fix cpplint errors in webrtc/api

webrtc/api/mediastreamproxy.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.
  */
 
 #ifndef WEBRTC_API_MEDIASTREAMPROXY_H_
 #define WEBRTC_API_MEDIASTREAMPROXY_H_
 
+#include <string>
+
 #include "webrtc/api/mediastreaminterface.h"
 #include "webrtc/api/proxy.h"
 
 namespace webrtc {
 
 BEGIN_SIGNALING_PROXY_MAP(MediaStream)
   PROXY_SIGNALING_THREAD_DESTRUCTOR()
   PROXY_CONSTMETHOD0(std::string, label)
   PROXY_METHOD0(AudioTrackVector, GetAudioTracks)
   PROXY_METHOD0(VideoTrackVector, GetVideoTracks)
   PROXY_METHOD1(rtc::scoped_refptr<AudioTrackInterface>,
                 FindAudioTrack,
                 const std::string&)
   PROXY_METHOD1(rtc::scoped_refptr<VideoTrackInterface>,
                 FindVideoTrack,
                 const std::string&)
   PROXY_METHOD1(bool, AddTrack, AudioTrackInterface*)
   PROXY_METHOD1(bool, AddTrack, VideoTrackInterface*)
   PROXY_METHOD1(bool, RemoveTrack, AudioTrackInterface*)
   PROXY_METHOD1(bool, RemoveTrack, VideoTrackInterface*)
   PROXY_METHOD1(void, RegisterObserver, ObserverInterface*)
   PROXY_METHOD1(void, UnregisterObserver, ObserverInterface*)
 END_PROXY_MAP()
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_MEDIASTREAMPROXY_H_