Support undoing offline page deletion

components/offline_pages/proto/offline_pages.proto

 // Copyright 2015 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.
 //
 // Offline page item protocol for storage and exchanging of offline page
 // metadata.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
@@ skipping 18 matching lines @@
   optional int64 file_size = 5;
 
   // Creation time of the offline archive.
   optional int64 creation_time = 6;
 
   // Last access time of the offline archive.
   optional int64 last_access_time = 7;
 
   // Number of times that the offline archive has been accessed.
   optional int32 access_count = 8;
+
+  // Flags about the state and behavior of the offline page.
+  enum Flags {
+    // No flag is set.
+    NONE = 0;
+    // Indicates that the page is marked for deletion. The real deletion will
+    // occur soon, after which the undo will not be possible.
+    MARKED_FOR_DELETION = 1;
+  };
+
+  // Flags for the offline page.
+  optional Flags flags = 9;
 }