Build WebRTC with data channel only.

webrtc/call/callfactory.h

Index: webrtc/call/callfactory.h
diff --git a/webrtc/call/callfactory.h b/webrtc/call/callfactory.h
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+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_CALL_CALLFACTORY_H_
+#define WEBRTC_CALL_CALLFACTORY_H_
+
+#include "webrtc/call/callfactoryinterface.h"
+
+namespace webrtc {
+
+class CallFactory : public CallFactoryInterface {
+  ~CallFactory() override {}
+
+  Call* CreateCall(const Call::Config& config) override;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_CALL_CALLFACTORY_H_