Use ArcFileSystemInstance to access files on ARC.

chrome/browser/chromeos/arc/fileapi/arc_file_system_util.h

+// 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.
+
+#ifndef CHROME_BROWSER_CHROMEOS_ARC_FILEAPI_ARC_FILE_SYSTEM_UTIL_H_
+#define CHROME_BROWSER_CHROMEOS_ARC_FILEAPI_ARC_FILE_SYSTEM_UTIL_H_
+
+#include "components/arc/common/file_system.mojom.h"
+#include "url/gurl.h"
+
+namespace arc {
+namespace file_system_util {
+
+// Utility which posts a task to run FileSystemInstance::GetFileSize.
+// This function must be called on the IO thread.
+void GetFileSizeOnIOThread(
+    const GURL& arc_url,
+    const mojom::FileSystemInstance::GetFileSizeCallback& callback);
+
+// Utility which posts a task to run FileSystemInstance::OpenFileToRead.
+// This function must be called on the IO thread.
+void OpenFileToReadOnIOThread(
+    const GURL& arc_url,
+    const mojom::FileSystemInstance::OpenFileToReadCallback& callback);
+
+}  // namespace file_system_util
+}  // namespace arc
+
+#endif  // CHROME_BROWSER_CHROMEOS_ARC_FILEAPI_ARC_FILE_SYSTEM_UTIL_H_