Reject the descriptions that attempt to change the order of m= section in current local description.

webrtc/pc/webrtcsession.h

 /*
  *  Copyright 2012 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.
  */
 
@@ skipping 43 matching lines @@
 class IceRestartAnswerLatch;
 class JsepIceCandidate;
 class MediaStreamSignaling;
 class RtcEventLog;
 class WebRtcSessionDescriptionFactory;
 
 extern const char kBundleWithoutRtcpMux[];
 extern const char kCreateChannelFailed[];
 extern const char kInvalidCandidates[];
 extern const char kInvalidSdp[];
-extern const char kMlineMismatch[];
+extern const char kMlineMismatchInAnswer[];
+extern const char kMlineMismatchInSubOffer[];

[Taylor Brandstetter, 2017/09/14 00:41:47]

nit: Might as well just spell out "Subsequent"

 extern const char kPushDownTDFailed[];
 extern const char kSdpWithoutDtlsFingerprint[];
 extern const char kSdpWithoutSdesCrypto[];
 extern const char kSdpWithoutIceUfragPwd[];
 extern const char kSdpWithoutSdesAndDtlsDisabled[];
 extern const char kSessionError[];
 extern const char kSessionErrorDesc[];
 extern const char kDtlsSrtpSetupFailureRtp[];
 extern const char kDtlsSrtpSetupFailureRtcp[];
 extern const char kEnableBundleFailed[];
@@ skipping 565 matching lines @@
 
 #ifdef HAVE_QUIC
   std::unique_ptr<QuicDataTransport> quic_data_transport_;
 #endif  // HAVE_QUIC
 
   RTC_DISALLOW_COPY_AND_ASSIGN(WebRtcSession);
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_PC_WEBRTCSESSION_H_