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

webrtc/api/ortc/mediadescription.h

Index: webrtc/api/ortc/mediadescription.h
diff --git a/webrtc/api/ortc/mediadescription.h b/webrtc/api/ortc/mediadescription.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_API_ORTC_MEDIADESCRIPTION_H_
+#define WEBRTC_API_ORTC_MEDIADESCRIPTION_H_
+
+#include <string>
+#include <utility>
+
+#include "webrtc/base/optional.h"
+
+namespace webrtc {
+
+// A structured representation of a media description within an SDP session
+// description.
+class MediaDescription {
+ public:
+  explicit MediaDescription(std::string mid) : mid_(std::move(mid)) {}
+
+  ~MediaDescription() {}
+
+  // The mid(media stream identification) is used for identifying media streams
+  // within a session description.
+  // https://tools.ietf.org/html/rfc5888#section-6
+  rtc::Optional<std::string> mid() const { return mid_; }
+  void set_mid(std::string mid) { mid_.emplace(std::move(mid)); }
+
+ private:
+  rtc::Optional<std::string> mid_;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_API_ORTC_MEDIADESCRIPTION_H_