Update API for Objective-C RTCSessionDescription.

webrtc/api/BUILD.gn

 # Copyright (c) 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.
 
 import("../build/webrtc.gni")
 
 if (is_ios) {
   source_set("rtc_api_objc") {
     deps = [
       "//webrtc/base:rtc_base_objc",
       #"//talk/libjingle:libjingle_peerconnection",
     ]
     cflags = [
       "-fobjc-arc",
       "-Wobjc-missing-property-synthesis",
     ]
     sources = [
       # Add RTCIceCandidate when there's a BUILD.gn for peer connection APIs
       #"objc/RTCIceCandidate+Private.h",
       #"objc/RTCIceCandidate.h",
       #"objc/RTCIceCandidate.mm",
       "objc/RTCIceServer+Private.h",
       "objc/RTCIceServer.h",
       "objc/RTCIceServer.mm",
+      "objc/RTCSessionDescription+Private.h",
+      "objc/RTCSessionDescription.h",
+      "objc/RTCSessionDescription.mm",
       "objc/WebRTC-Prefix.pch",
     ]
   }
 }