Always use arc::InstanceHolder<T>::GetInstanceForMethod

chrome/browser/chromeos/arc/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/arc_downloads_watcher_service.h"
 
 #include <map>
 #include <memory>
 #include <utility>
 
@@ skipping 270 matching lines @@
   if (watcher_) {
     BrowserThread::DeleteSoon(BrowserThread::FILE, FROM_HERE,
                               watcher_.release());
   }
 }
 
 void ArcDownloadsWatcherService::OnDownloadsChanged(
     mojo::Array<mojo::String> paths) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
-  auto* instance = arc_bridge_service()->file_system()->instance();
+  auto* instance = arc_bridge_service()->file_system()->GetInstanceForMethod(
+      "RequestMediaScan");
   if (!instance)
     return;
   instance->RequestMediaScan(std::move(paths));
 }
 
 }  // namespace arc