Fix some warnings in ObjC code.

webrtc/api/objc/RTCDataChannel.mm

Index: webrtc/api/objc/RTCDataChannel.mm
diff --git a/webrtc/api/objc/RTCDataChannel.mm b/webrtc/api/objc/RTCDataChannel.mm
index 98599058ee8ebec28ba52f096ad3a816c56b8034..4be3a227becc54776d975e9b6ec927c407f509d7 100644
--- a/webrtc/api/objc/RTCDataChannel.mm
+++ b/webrtc/api/objc/RTCDataChannel.mm
@@ -34,8 +34,8 @@ class DataChannelDelegateAdapter : public DataChannelObserver {
 
   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];
     }
   }