Convert drive::FileWriteHelper to a single function for simplification.

chrome/browser/chromeos/drive/write_on_cache_file.h

+// Copyright (c) 2012 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_DRIVE_WRITE_ON_CACHE_FILE_H_
+#define CHROME_BROWSER_CHROMEOS_DRIVE_WRITE_ON_CACHE_FILE_H_
+
+#include "base/callback_forward.h"
+#include "chrome/browser/chromeos/drive/file_errors.h"
+
+namespace base {
+class FilePath;
+}
+
+namespace drive {
+
+class FileSystemInterface;
+
+// Callback type for WriteOnCacheFile.
+typedef base::Callback<void (FileError, const base::FilePath& path)>
+    WriteOnCacheFileCallback;
+
+// Creates (if needed) a file at |path|, brings it to the local cache,
+// and invokes |callback| on blocking thread pool with the cache file path.
+// The |callback| can write to the file at the path by normal local file I/O
+// operations. After it returns, the written content is synced to the server.
+//
+// Must be called from UI thread.
+void WriteOnCacheFile(FileSystemInterface* file_system,
+                      const base::FilePath& path,
+                      const WriteOnCacheFileCallback& callback);
+
+}  // namespace drive
+
+#endif  // CHROME_BROWSER_CHROMEOS_DRIVE_WRITE_ON_CACHE_FILE_H_