Support building WebRTC without audio and video for IOS

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCPeerConnection+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 37 matching lines @@
 
   void OnIceCandidatesRemoved(
       const std::vector<cricket::Candidate>& candidates) override;
 
  private:
   __weak RTCPeerConnection *peer_connection_;
 };
 
 } // namespace webrtc
 
-
 @interface RTCPeerConnection ()
 
 /** The native PeerConnectionInterface created during construction. */
 @property(nonatomic, readonly)
     rtc::scoped_refptr<webrtc::PeerConnectionInterface> nativePeerConnection;
 
 /** Initialize an RTCPeerConnection with a configuration, constraints, and
  *  delegate.
  */
 - (instancetype)initWithFactory:
@@ skipping 29 matching lines @@
     (webrtc::PeerConnectionInterface::IceGatheringState)nativeState;
 
 + (NSString *)stringForIceGatheringState:(RTCIceGatheringState)state;
 
 + (webrtc::PeerConnectionInterface::StatsOutputLevel)
     nativeStatsOutputLevelForLevel:(RTCStatsOutputLevel)level;
 
 @end
 
 NS_ASSUME_NONNULL_END