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

webrtc/api/ortc/sessiondescription.h

Index: webrtc/api/ortc/sessiondescription.h
diff --git a/webrtc/api/ortc/sessiondescription.h b/webrtc/api/ortc/sessiondescription.h
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+++ b/webrtc/api/ortc/sessiondescription.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_SESSIONDESCRIPTION_H_
+#define WEBRTC_API_ORTC_SESSIONDESCRIPTION_H_
+
+#include <string>
+
+namespace webrtc {
+
+// A structured representation of an SDP message.
+class SessionDescription {
+ public:
+  SessionDescription(std::string session_id, std::string session_version)
+      : session_id_(session_id), session_version_(session_version) {}

[Taylor Brandstetter, 2017/03/11 00:24:29]

Should be session_id_(std::move(session_id)) to ensure an extra copy isn't made.
Same deal with the setters.

[Zhi Huang, 2017/03/11 01:20:39]

Done.

+
+  std::string session_id() const { return session_id_; }

[Taylor Brandstetter, 2017/03/11 00:24:29]

Would be nice to add comments for where these things come from. session_id is
the "sess-id" field from the o= line, and "session_version" is the
"sess-version" field.

[Taylor Brandstetter, 2017/03/11 00:24:29]

These could return const refs.

[Zhi Huang, 2017/03/11 01:20:39]

Done.

[Zhi Huang, 2017/03/11 01:20:39]

Done.

+  void set_session_id(std::string session_id) { session_id_ = session_id; }
+
+  std::string session_version() const { return session_version_; }
+  void set_session_version(std::string session_version) {
+    session_version_ = session_version;
+  }
+
+ private:
+  std::string session_id_;
+  std::string session_version_;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_API_ORTC_SESSIONDESCRIPTION_H_