[Thumbnails DB] Add functionality to clear unused on-demand favicons.

components/history/core/browser/thumbnail_database.h

Index: components/history/core/browser/thumbnail_database.h
diff --git a/components/history/core/browser/thumbnail_database.h b/components/history/core/browser/thumbnail_database.h
index 697faf3abec0a6d18d3dc8639ae869e732116ad8..2fbeb15453961399fe1455c7e4886b5ab570bdf9 100644
--- a/components/history/core/browser/thumbnail_database.h
+++ b/components/history/core/browser/thumbnail_database.h
@@ -66,6 +66,13 @@ class ThumbnailDatabase {
   // true try to trim all unused cache, otherwise trim by half.
   void TrimMemory(bool aggressively);
 
+  // Delete all on-demand favicon bitmaps that have not been requested long
+  // (i.e. not after |expiration_threshold|). Do not delete bitmaps for URLs
+  // that are bookmarked in |bookmark_model|. Furthermore, delete all favicons
+  // with no corresponding bitmap and all mappings with no corresponding
+  // favicon.
+  void ClearOldOnDemandFavicons(base::Time expiration_threshold);
+
   // Favicon Bitmaps -----------------------------------------------------------
 
   // Returns true if there are favicon bitmaps for |icon_id|. If