Fix some warnings in ObjC code.

BUG=

Review URL: https://codereview.webrtc.org/1824573002

Cr-Commit-Position: refs/heads/master@{#12076}
diff --git a/webrtc/api/objc/RTCDataChannel.h b/webrtc/api/objc/RTCDataChannel.h
index 8ab4b34..43f5767 100644
--- a/webrtc/api/objc/RTCDataChannel.h
+++ b/webrtc/api/objc/RTCDataChannel.h
@@ -46,7 +46,7 @@
 
 /** The data channel's |bufferedAmount| changed. */
 - (void)dataChannel:(RTCDataChannel *)dataChannel
-    didChangeBufferedAmount:(NSUInteger)amount;
+    didChangeBufferedAmount:(uint64_t)amount;
 
 @end