Use OnceCallback for Mojo binding connection error handlers.

mojo/public/cpp/bindings/lib/connector.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/public/cpp/bindings/connector.h"
 
 #include <stdint.h>
-#include <utility>
 
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/synchronization/lock.h"
@@ skipping 453 matching lines @@
     message_pipe_ = std::move(dummy_pipe.handle0);
   } else {
     CancelWait();
   }
 
   if (force_async_handler) {
     if (!paused_)
       WaitToReadMore();
   } else {
     error_ = true;
-    if (!connection_error_handler_.is_null())
-      connection_error_handler_.Run();
+    if (connection_error_handler_)
+      std::move(connection_error_handler_).Run();
   }
 }
 
 void Connector::EnsureSyncWatcherExists() {
   if (sync_watcher_)
     return;
   sync_watcher_.reset(new SyncHandleWatcher(
       message_pipe_.get(), MOJO_HANDLE_SIGNAL_READABLE,
       base::Bind(&Connector::OnSyncHandleWatcherHandleReady,
                  base::Unretained(this))));
 }
 
 }  // namespace mojo