Omnibox: Rebuild HQP Cache if it's old

chrome/browser/history/in_memory_url_index_cache.proto

 // Copyright (c) 2011 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.
 //
 // InMemoryURLIndex caching protocol buffers.
 //
 // At certain times during browser operation, the indexes from the
 // InMemoryURLIndex are written to a disk-based cache using the
 // following protobuf description.
 
@@ skipping 66 matching lines @@
     message WordStartsMapEntry {
       required int64 history_id = 1;
       repeated int32 url_word_starts = 2 [packed=true];
       repeated int32 title_word_starts = 3 [packed=true];
     }
 
     required uint32 item_count = 1;
     repeated WordStartsMapEntry word_starts_map_entry = 2;
   }
 
-  required int64 timestamp = 1;
+  // The date that the cache was last rebuilt from history.  Note that
+  // this cache may include items that were visited after this date if
+  // the InMemoryURLIndex was updated on the fly.  This timestamp is meant
+  // to indicate the last date the index was rebuilt from the ground truth:
+  // the history database on disk.
+  required int64 last_rebuild_timestamp = 1;
   // If there is no version we'll assume version 0.
   optional int32 version = 2;
   required int32 history_item_count = 3;
 
   optional WordListItem word_list = 4;
   optional WordMapItem word_map = 5;
   optional CharWordMapItem char_word_map = 6;
   optional WordIDHistoryMapItem word_id_history_map = 7;
   optional HistoryInfoMapItem history_info_map = 8;
   optional WordStartsMapItem word_starts_map = 9;
 }