arc: Use GET_INTERFACE_FOR_METHOD macro

chrome/browser/chromeos/arc/downloads_watcher/arc_downloads_watcher_service.cc

 // Copyright 2016 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 "chrome/browser/chromeos/arc/downloads_watcher/arc_downloads_watcher_service.h"
 
 #include <string.h>
 
 #include <algorithm>
 #include <iterator>
@@ skipping 342 matching lines @@
   if (watcher_) {
     BrowserThread::DeleteSoon(BrowserThread::FILE, FROM_HERE,
                               watcher_.release());
   }
 }
 
 void ArcDownloadsWatcherService::OnDownloadsChanged(
     const std::vector<std::string>& paths) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  auto* instance = arc_bridge_service()->file_system()->GetInstanceForMethod(
-      "RequestMediaScan");
+  auto* instance = GET_INSTANCE_FOR_METHOD(arc_bridge_service()->file_system(),
+                                           RequestMediaScan);
   if (!instance)
     return;
   instance->RequestMediaScan(paths);
 }
 
 }  // namespace arc