Create empty PayloadTypeMapper files and add them to builds.

webrtc/media/engine/payload_type_mapper.h

Index: webrtc/media/engine/payload_type_mapper.h
diff --git a/webrtc/modules/audio_coding/neteq/packet.cc b/webrtc/media/engine/payload_type_mapper.h
similarity index 66%
copy from webrtc/modules/audio_coding/neteq/packet.cc
copy to webrtc/media/engine/payload_type_mapper.h
index 8a19fe4d5923d5e22a5b485600c6da0c5f3856ba..d1f8f35949adbf8fe68189264b305b0405958c83 100644
--- a/webrtc/modules/audio_coding/neteq/packet.cc
+++ b/webrtc/media/engine/payload_type_mapper.h
@@ -8,12 +8,9 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "webrtc/modules/audio_coding/neteq/packet.h"
+#ifndef WEBRTC_MEDIA_ENGINE_PAYLOAD_TYPE_MAPPER_H_
+#define WEBRTC_MEDIA_ENGINE_PAYLOAD_TYPE_MAPPER_H_
 
-namespace webrtc {
+// This file intentionally left blank for build purposes.
 
-Packet::Packet() = default;
-
-Packet::~Packet() = default;
-
-}  // namespace webrtc
+#endif  // WEBRTC_MEDIA_ENGINE_PAYLOAD_TYPE_MAPPER_H_