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

webrtc/api/ortc/mediadescription_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/mediadescription.h"
+#include "webrtc/test/gtest.h"
+
+namespace webrtc {
+
+class MediaDescriptionTest : public testing::Test {};
+
+TEST_F(MediaDescriptionTest, CreateMediaDescription) {
+  MediaDescription m("a");
+  EXPECT_EQ("a", m.mid());
+}
+}