Update API for Objective-C RTCPeerConnection.

webrtc/api/objc/RTCSessionDescription.mm

 /*
  *  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 38 matching lines @@
   if (!description) {
     RTCLogError(@"Failed to create session description: %s\nline: %s",
                 error.description.c_str(),
                 error.line.c_str());
   }
 
   return description;
 }
 
 - (instancetype)initWithNativeDescription:
-    (webrtc::SessionDescriptionInterface *)nativeDescription {
+    (const webrtc::SessionDescriptionInterface *)nativeDescription {
   NSParameterAssert(nativeDescription);
   std::string sdp;
   nativeDescription->ToString(&sdp);
   RTCSdpType type = [[self class] typeForString:nativeDescription->type()];
 
   return [self initWithType:type
                         sdp:[NSString stringForStdString:sdp]];
 }
 
 + (std::string)stringForType:(RTCSdpType)type {
@@ skipping 14 matching lines @@
     return RTCSdpTypePrAnswer;
   } else if (string == webrtc::SessionDescriptionInterface::kAnswer) {
     return RTCSdpTypeAnswer;
   } else {
     RTC_NOTREACHED();
     return RTCSdpTypeOffer;
   }
 }
 
 @end