Make chrome.filesystem API fully work with the Chrome OS Google Drive folder.

chrome/browser/extensions/api/file_system/file_system_api.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_EXTENSIONS_API_FILE_SYSTEM_FILE_SYSTEM_API_H_
 #define CHROME_BROWSER_EXTENSIONS_API_FILE_SYSTEM_FILE_SYSTEM_API_H_
 
 #include "chrome/browser/extensions/extension_function.h"
 #include "chrome/common/extensions/api/file_system.h"
 #include "ui/shell_dialogs/select_file_dialog.h"
@@ skipping 14 matching lines @@
  protected:
   enum EntryType {
     READ_ONLY,
     WRITABLE
   };
 
   virtual ~FileSystemEntryFunction() {}
 
   bool HasFileSystemWritePermission();
 
-  // Called on the FILE thread. This is called when a writable file entry is
-  // being returned. The function will ensure the file exists, creating it if
-  // necessary, and also check that the file is not a link.
+  // This is called when a writable file entry is being returned. The function
+  // will ensure the file exists, creating it if necessary, and also check that
+  // the file is not a link. If it succeeds it proceeds to
+  // RegisterFileSystemAndSendResponse, otherwise to HandleWritableFileError.
   void CheckWritableFile(const base::FilePath& path);
 
   // This will finish the choose file process. This is either called directly
   // from FileSelected, or from CreateFileIfNecessary. It is called on the UI
   // thread.
   void RegisterFileSystemAndSendResponse(const base::FilePath& path,
                                          EntryType entry_type);
 
   // called on the UI thread if there is a problem checking a writable file.
   void HandleWritableFileError();
@@ skipping 57 matching lines @@
 
  private:
   // FileSelected and FileSelectionCanceled are called by the file picker.
   void FileSelected(const base::FilePath& path, EntryType entry_type);
   void FileSelectionCanceled();
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_FILE_SYSTEM_FILE_SYSTEM_API_H_