Build WebRTC with data channel only.

webrtc/pc/nullaudiofactory.cc

Index: webrtc/pc/nullaudiofactory.cc
diff --git a/webrtc/pc/nullaudiofactory.cc b/webrtc/pc/nullaudiofactory.cc
new file mode 100644
index 0000000000000000000000000000000000000000..be71f27f882c1ad902a4d5bf90bd711f77d62937
--- /dev/null
+++ b/webrtc/pc/nullaudiofactory.cc
@@ -0,0 +1,25 @@
+/*
+ *  Copyright 2017 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.
+ */
+
+#include "webrtc/api/audio_codecs/audio_decoder_factory.h"
+#include "webrtc/api/audio_codecs/audio_encoder_factory.h"
+#include "webrtc/base/scoped_ref_ptr.h"
+
+namespace webrtc {
+
+rtc::scoped_refptr<AudioEncoderFactory> CreateBuiltinAudioEncoderFactoryImpl() {
+  return rtc::scoped_refptr<AudioEncoderFactory>();
+}
+
+rtc::scoped_refptr<AudioDecoderFactory> CreateBuiltinAudioDecoderFactoryImpl() {
+  return rtc::scoped_refptr<AudioDecoderFactory>();
+}
+
+}  // namespace webrtc