Add SdpHelper for more reliable codec configuration in WebrtcTransport.

remoting/protocol/sdp_message.h

Index: remoting/protocol/sdp_message.h
diff --git a/remoting/protocol/sdp_message.h b/remoting/protocol/sdp_message.h
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+++ b/remoting/protocol/sdp_message.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef REMOTING_PROTOCOL_SDP_MESSAGE_H_
+#define REMOTING_PROTOCOL_SDP_MESSAGE_H_
+
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+
+namespace remoting {
+namespace protocol {
+
+// SdpMessage is used to process session descriptions messages in SDP format
+// generated by WebRTC (see RFC 4566). In particularly it allows configuring
+// video and audio codecs.
+//
+// It also normalizes the SDP message to make sure the text used for HMAC
+// signature verification is the same that was signed on the sending side.
+// This is necessary because WebRTC generates SDP with CRLF line endings which
+// are sometimes converted to LF after passing the signaling channel.
+class SdpMessage {
+ public:
+  explicit SdpMessage(const std::string& sdp);
+  ~SdpMessage();
+
+  bool has_audio() const { return has_audio_; }
+  bool has_video() const { return has_video_; }
+
+  // Returns string representation of the SDP message. The result always has
+  // line-endings normalized to LF and empty lines removed.
+  std::string ToString() const;
+
+  // Adds specified parameters for the |codec|. Returns false if the |codec| is
+  // not listed anywhere in the SDP message.
+  bool AddCodecParameter(const std::string& codec,
+                         const std::string& parameters_to_add);
+
+ private:
+  // Finds the line of the form "a=rtpmap:<payload_type> <codec>/.." with the
+  // specified |codec|. Sets |line_num| to line number and |payload_type| to the
+  // payload type from that line. Returns false if the codec wasn't found.
+  bool FindCodec(const std::string& codec,
+                 int* line_num,
+                 std::string* payload_type) const;
+
+  std::vector<std::string> sdp_lines_;
+
+  bool has_audio_ = false;
+  bool has_video_ = false;
+
+  DISALLOW_COPY_AND_ASSIGN(SdpMessage);
+};
+
+}  // namespace protocol
+}  // namespace remoting
+
+#endif  // REMOTING_PROTOCOL_SDP_MESSAGE_H_