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

chrome/browser/chromeos/drive/file_system_util.h

Index: chrome/browser/chromeos/drive/file_system_util.h
diff --git a/chrome/browser/chromeos/drive/file_system_util.h b/chrome/browser/chromeos/drive/file_system_util.h
index e9879ad54fc46403fcc5c7af63cf8334e00258d7..8ad96dd15266be23a2d4770864eab2ede035c09f 100644
--- a/chrome/browser/chromeos/drive/file_system_util.h
+++ b/chrome/browser/chromeos/drive/file_system_util.h
@@ -151,19 +151,16 @@ void MigrateCacheFilesFromOldDirectories(
 
 // Callback type for PrepareWritableFileAndRun.
 typedef base::Callback<void (FileError, const base::FilePath& path)>
-    OpenFileCallback;
+    PrepareWritableFileCallback;
 
 // Invokes |callback| on blocking thread pool, after converting virtual |path|
 // string like "/special/drive/foo.txt" to the concrete local cache file path.
 // After |callback| returns, the written content is synchronized to the server.
 //
-// If |path| is not a Drive path, it is regarded as a local path and no path
-// conversion takes place.
-//
-// Must be called from UI thread.
+// The |path| must be a path under Drive. Must be called from UI thread.
 void PrepareWritableFileAndRun(Profile* profile,
                                const base::FilePath& path,
-                               const OpenFileCallback& callback);
+                               const PrepareWritableFileCallback& callback);
 
 // Ensures the existence of |directory| of '/special/drive/foo'.  This will
 // create |directory| and its ancestors if they don't exist.  |callback| is