Add support for persistent file access in apps.

chrome/common/extensions/api/file_system.idl

 // 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.
 
 namespace fileSystem {
   dictionary AcceptOption {
     // This is the optional text description for this option. If not present,
     // a description will be automatically generated; typically containing an
     // expanded list of valid extensions (e.g. "text/html" may expand to
     // "*.html, *.htm").
@@ skipping 37 matching lines @@
     AcceptOption[]? accepts;
 
     // Whether to accept all file types, in addition to the options specified
     // in the accepts argument. The default is true. If the accepts field is
     // unset or contains no valid entries, this will always be reset to true.
     boolean? acceptsAllTypes;
   };
   callback GetDisplayPathCallback = void (DOMString displayPath);
   callback FileEntryCallback = void ([instanceOf=FileEntry] object fileEntry);
   callback IsWritableCallback = void (boolean isWritable);
+  callback IsRestorableCallback = void (boolean isRestorable);
 
   interface Functions {
     // Get the display path of a FileEntry object. The display path is based on
     // the full path of the file on the local file system, but may be made more
     // readable for display purposes.
     static void getDisplayPath([instanceOf=FileEntry] object fileEntry,
                                GetDisplayPathCallback callback);
 
     // Get a writable FileEntry from another FileEntry. This call will fail if
     // the application does not have the 'write' permission under 'fileSystem'.
     static void getWritableEntry([instanceOf=FileEntry] object fileEntry,
                                  FileEntryCallback callback);
 
     // Gets whether this FileEntry is writable or not.
     static void isWritableEntry([instanceOf=FileEntry] object fileEntry,
                                 IsWritableCallback callback);
 
     // Ask the user to choose a file.
     static void chooseEntry(optional ChooseEntryOptions options,
                             FileEntryCallback callback);
 
-    // Returns the file entry with the given id.
-    [nocompile] static FileEntry getEntryById(DOMString id);
+    // Returns the file entry with the given id if it can be restored. This call
+    // will fail otherwise.
+    static void restoreEntry(DOMString id, FileEntryCallback callback);
 
-    // Returns the id of the given file entry. This can be used to retrieve file
-    // entries with getEntryById(). When an app is restarted (ie: it is sent the
-    // onRestarted event) it can regain access to the file entries it had by
-    // remembering their ids and calling getEntryById().
-    [nocompile] static DOMString getEntryId(
-        [instanceOf=FileEntry] object fileEntry);
+    // Returns whether a file entry for the given id can be restored, i.e.
+    // whether restoreEntry would succeed with this id now.
+    static void isRestorable(DOMString id, IsRestorableCallback callback);
+
+    // Returns an id that can be passed to restoreEntry to regain access to a
+    // given file entry. Only the 500 most recently used entries are retained,
+    // where calls to retainEntry and restoreEntry count as use.
+    static DOMString retainEntry([instanceOf=FileEntry] object fileEntry);
   };
 };