Build WebRTC with data channel only.

webrtc/media/base/codec_video_nullimpl.cc

+/*
+ *  Copyright (c) 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/media/base/codec.h"
+
+namespace cricket {
+bool IsSameH264Profile(const CodecParameterMap& params1,
+                       const CodecParameterMap& params2) {
+  return false;

[Taylor Brandstetter, 2017/05/18 17:57:05]

I think it would be simpler to just move "profile_level_id" to
webrtc/media/base. But if you don't, this should have an RTC_NOTREACHED.

[Zhi Huang, 2017/05/23 03:40:35]

Yes, this should make it cleaner.

+}
+}