[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..0a9ea8e725a2dff06a32357f311f33ddac543ed2 100644
--- a/components/history/core/browser/thumbnail_database.h
+++ b/components/history/core/browser/thumbnail_database.h
@@ -66,6 +66,11 @@ class ThumbnailDatabase {
   // true try to trim all unused cache, otherwise trim by half.
   void TrimMemory(bool aggressively);
 
+  // Get all on-demand favicon bitmaps that have been last requested prior to
+  // |threshold|.
+  std::map<favicon_base::FaviconID, IconMappingsForExpiry>
+  GetOldOnDemandFavicons(base::Time threshold);
+
   // Favicon Bitmaps -----------------------------------------------------------
 
   // Returns true if there are favicon bitmaps for |icon_id|. If
@@ -200,6 +205,10 @@ class ThumbnailDatabase {
   // Returns true if the deletion succeeded.
   bool DeleteIconMappings(const GURL& page_url);
 
+  // Deletes the icon mapping entries for the given favicon.

[pkotwicz, 2017/07/10 19:50:19]

Nit: "given favicon" -> "given favicon ID"

[jkrcal, 2017/07/11 13:19:35]

Done.

+  // Returns true if the deletion succeeded.
+  bool DeleteIconMappingsForFaviconId(favicon_base::FaviconID id);
+
   // Deletes the icon mapping with |mapping_id|.
   // Returns true if the deletion succeeded.
   bool DeleteIconMapping(IconMappingID mapping_id);