Starting with patch 34 from https://codereview.webrtc.org/2854123003/

webrtc/call/callfactoryinterface.h

Index: webrtc/call/callfactoryinterface.h
diff --git a/webrtc/call/callfactoryinterface.h b/webrtc/call/callfactoryinterface.h
new file mode 100644
index 0000000000000000000000000000000000000000..6a0329a3d0c510f68727c972f9255f46380b9abf
--- /dev/null
+++ b/webrtc/call/callfactoryinterface.h
@@ -0,0 +1,34 @@
+/*
+ *  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_CALLFACTORYINTERFACE_H_
+#define WEBRTC_CALL_CALLFACTORYINTERFACE_H_
+
+#include <memory>
+
+#include "webrtc/call/call.h"
+
+namespace webrtc {
+
+// This interface exists to allow webrtc to be optionally built without media
+// support (i.e., if only being used for data channels). PeerConnectionFactory
+// is constructed with a CallFactoryInterface, which may or may not be null.
+class CallFactoryInterface {
+ public:
+  virtual ~CallFactoryInterface() {}
+
+  virtual Call* CreateCall(const Call::Config& config) = 0;
+};
+
+std::unique_ptr<CallFactoryInterface> CreateCallFactory();
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_CALL_CALLFACTORYINTERFACE_H_