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

webrtc/api/ortc/sessiondescription_unittest.cc

+/*
+ *  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"
+#include "webrtc/test/gtest.h"
+
+namespace webrtc {
+
+class SessionDescriptionTest : public testing::Test {};
+
+TEST_F(SessionDescriptionTest, CreateSessionDescription) {
+  SessionDescription s("a", "0");
+  EXPECT_EQ("a", s.session_id());
+  EXPECT_EQ("0", s.session_version());
+}
+}