Add API to get a folder's size.

chrome/common/extensions/api/file_manager_private_internal.idl

 // Copyright 2014 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.
 
 // Internal, used by fileManagerPrivate's custom bindings.
 [platforms=("chromeos"),
  implemented_in="chrome/browser/chromeos/extensions/file_manager/file_manager_private_api_functions.h"]
 namespace fileManagerPrivateInternal {
   // Entry information that renderers need to create an Entry instance.
   dictionary EntryDescription {
@@ skipping 71 matching lines @@
                           DOMString newName,
                           StartCopyCallback callback);
     static void zipSelection(DOMString parentUrl,
                              DOMString[] urls,
                              DOMString destName,
                              ZipSelectionCallback callback);
     static void validatePathNameLength(
         DOMString parentUrl,
         DOMString name,
         ValidatePathNameLengthCallback callback);
+    static void getDirectorySize(DOMString url,
+                                 SimpleCallback callback);
   };
 };