Create empty PayloadTypeMapper files and add them to builds.

webrtc/media/media.gyp

 # Copyright (c) 2016 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.
 
 {
   'includes': [ '../build/common.gypi', ],
@@ skipping 51 matching lines @@
         'base/videocommon.cc',
         'base/videocommon.h',
         'base/videoframe.cc',
         'base/videoframe.h',
         'base/videoframefactory.cc',
         'base/videoframefactory.h',
         'base/videosourcebase.cc',
         'base/videosourcebase.h',
         'devices/videorendererfactory.h',
         'engine/nullwebrtcvideoengine.h',
+        'engine/payload_type_mapper.cc',
+        'engine/payload_type_mapper.h',
         'engine/simulcast.cc',
         'engine/simulcast.h',
         'engine/webrtccommon.h',
         'engine/webrtcmediaengine.cc',
         'engine/webrtcmediaengine.h',
         'engine/webrtcmediaengine.cc',
         'engine/webrtcvideocapturer.cc',
         'engine/webrtcvideocapturer.h',
         'engine/webrtcvideocapturerfactory.h',
         'engine/webrtcvideocapturerfactory.cc',
@@ skipping 270 matching lines @@
               'sources': [
                 'rtc_media_unittests.isolate',
               ],
             },
           ],
         }],
       ],  # conditions
     }],  # include_tests==1
   ],  # conditions
 }