Fix some warnings in ObjC code.

webrtc/api/objc/RTCDataChannel.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 16 matching lines @@
 
   void OnMessage(const DataBuffer& buffer) override {
     RTCDataBuffer *data_buffer =
         [[RTCDataBuffer alloc] initWithNativeBuffer:buffer];
     [channel_.delegate dataChannel:channel_
        didReceiveMessageWithBuffer:data_buffer];
   }
 
   void OnBufferedAmountChange(uint64_t previousAmount) override {
     id<RTCDataChannelDelegate> delegate = channel_.delegate;
-    if ([delegate
-            respondsToSelector:@selector(channel:didChangeBufferedAmount:)]) {
+    SEL sel = @selector(dataChannel:didChangeBufferedAmount:);
+    if ([delegate respondsToSelector:sel]) {
       [delegate dataChannel:channel_ didChangeBufferedAmount:previousAmount];
     }
   }
 
  private:
   __weak RTCDataChannel *channel_;
 };
 }
 
 
@@ skipping 165 matching lines @@
     case RTCDataChannelStateOpen:
       return @"Open";
     case RTCDataChannelStateClosing:
       return @"Closing";
     case RTCDataChannelStateClosed:
       return @"Closed";
   }
 }
 
 @end