Add skeleton webrtc::SessionDescription and webrtc::MediaDescription classes.

webrtc/api/ortc/sessiondescription.cc

Index: webrtc/api/ortc/sessiondescription.cc
diff --git a/webrtc/api/ortc/sessiondescription.cc b/webrtc/api/ortc/sessiondescription.cc
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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.
+ */
+
+#include "webrtc/api/ortc/sessiondescription.h"
+
+namespace webrtc {}