Omnibox: Rebuild HQP Cache if it's old

chrome/browser/history/in_memory_url_index_cache.proto

Index: chrome/browser/history/in_memory_url_index_cache.proto
diff --git a/chrome/browser/history/in_memory_url_index_cache.proto b/chrome/browser/history/in_memory_url_index_cache.proto
index b5cf41b2a2d387e5cc103302eadf3ac69c3d36bc..3a39e746bf334d738fe0a61462b28eaf59685b64 100644
--- a/chrome/browser/history/in_memory_url_index_cache.proto
+++ b/chrome/browser/history/in_memory_url_index_cache.proto
@@ -84,7 +84,12 @@ message InMemoryURLIndexCacheItem {
     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;