Update API for Objective-C RTCPeerConnection.

webrtc/api/objc/RTCSessionDescription+Private.h

 /*
  *  Copyright 2015 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 12 matching lines @@
  */
 @property(nonatomic, readonly)
     webrtc::SessionDescriptionInterface *nativeDescription;
 
 /**
  * Initialize an RTCSessionDescription from a native
  * SessionDescriptionInterface. No ownership is taken of the native session
  * description.
  */
 - (instancetype)initWithNativeDescription:
-    (webrtc::SessionDescriptionInterface *)nativeDescription;
+    (const webrtc::SessionDescriptionInterface *)nativeDescription;
 
 + (std::string)stringForType:(RTCSdpType)type;
 
 + (RTCSdpType)typeForString:(const std::string &)string;
 
 @end
 
 NS_ASSUME_NONNULL_END