Fix some sign-compare warnings in webrtc/api.

webrtc/api/datachannel.cc

 /*
  *  Copyright 2012 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 366 matching lines @@
         Close();
       }
 
       return;
     }
     queued_received_data_.Push(buffer.release());
   }
 }
 
 void DataChannel::OnStreamClosedRemotely(uint32_t sid) {
-  if (data_channel_type_ == cricket::DCT_SCTP && sid == config_.id) {
+  if (data_channel_type_ == cricket::DCT_SCTP &&
+      sid == static_cast<uint32_t>(config_.id)) {
     Close();
   }
 }
 
 void DataChannel::OnChannelReady(bool writable) {
   writable_ = writable;
   if (!writable) {
     return;
   }
 
@@ skipping 232 matching lines @@
     QueueControlMessage(buffer);
   } else {
     LOG(LS_ERROR) << "Closing the DataChannel due to a failure to send"
                   << " the CONTROL message, send_result = " << send_result;
     Close();
   }
   return retval;
 }
 
 }  // namespace webrtc