Limit the number of history urls indexed for omnibox suggestions.

components/history/core/browser/url_database.h

 // 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.
 
 #ifndef COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_
 
 #include <stddef.h>
 
 #include "base/macros.h"
@@ skipping 132 matching lines @@
     bool GetNextURL(URLRow* r);
 
    private:
     DISALLOW_COPY_AND_ASSIGN(URLEnumerator);
   };
 
   // Initializes the given enumerator to enumerator all URLs in the database.
   bool InitURLEnumeratorForEverything(URLEnumerator* enumerator);
 
   // Initializes the given enumerator to enumerator all URLs in the database
-  // that are historically significant: ones having been visited within 3 days,
-  // having their URL manually typed more than once, or having been visited
-  // more than 3 times.
+  // that are historically significant: ones having their URL manually typed
+  // more than once, having been visited within 3 days, or having been visited
+  // more than 3 times in the order of the most significant ones first.
   bool InitURLEnumeratorForSignificant(URLEnumerator* enumerator);
 
   // Autocomplete --------------------------------------------------------------
 
   // Fills the given array with URLs matching the given prefix.  They will be
   // sorted by typed count, then by visit count, then by visit date (most recent
   // first) up to the given maximum number.  If |typed_only| is true, only urls
   // that have been typed once are returned.  For caller convenience, returns
   // whether any results were found.
   bool AutocompleteForPrefix(const std::string& prefix,
@@ skipping 163 matching lines @@
 // is_null() then this function determines a new time threshold each time it is
 // called. Since getting system time can be costly (such as for cases where
 // this function will be called in a loop over many history items), you can
 // provide a non-null |time_cache| by simply initializing |time_cache| with
 // AutocompleteAgeThreshold() (or any other desired time in the past).
 bool RowQualifiesAsSignificant(const URLRow& row, const base::Time& threshold);
 
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_