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

components/history/core/browser/expire_history_backend.h

Index: components/history/core/browser/expire_history_backend.h
diff --git a/components/history/core/browser/expire_history_backend.h b/components/history/core/browser/expire_history_backend.h
index ebffc5b62e4e14ee9b45b271c18c0a4f1333b326..e2fb4f848b2cb1cbd0c390d8abe36dc68ef89041 100644
--- a/components/history/core/browser/expire_history_backend.h
+++ b/components/history/core/browser/expire_history_backend.h
@@ -20,6 +20,7 @@ class GURL;
 class TestingProfile;
 
 namespace base {
+struct Feature;
 class SequencedTaskRunner;
 }
 
@@ -42,6 +43,11 @@ class ExpiringVisitsReader {
 
 typedef std::vector<const ExpiringVisitsReader*> ExpiringVisitsReaders;
 
+namespace internal {
+// Feature that enables clearing old on-demand favicons.
+extern const base::Feature kClearOldOnDemandFavicons;
+}  // namespace internal
+
 // Helper component to HistoryBackend that manages expiration and deleting of
 // history.
 //
@@ -102,6 +108,10 @@ class ExpireHistoryBackend {
   FRIEND_TEST_ALL_PREFIXES(ExpireHistoryTest, ExpireSomeOldHistory);
   FRIEND_TEST_ALL_PREFIXES(ExpireHistoryTest, ExpiringVisitsReader);
   FRIEND_TEST_ALL_PREFIXES(ExpireHistoryTest, ExpireSomeOldHistoryWithSource);
+  FRIEND_TEST_ALL_PREFIXES(ExpireHistoryTest,
+                           ClearOldOnDemandFaviconsDoesNotDeleteStarred);
+  FRIEND_TEST_ALL_PREFIXES(ExpireHistoryTest,
+                           ClearOldOnDemandFaviconsDoesDeleteUnstarred);
   friend class ::TestingProfile;
 
   struct DeleteEffects {
@@ -206,6 +216,9 @@ class ExpireHistoryBackend {
   // future.
   void DoExpireIteration();
 
+  // Clears all old on-demand favicons from thumbnail database.
+  void ClearOldOnDemandFavicons(base::Time expiration_threshold);
+
   // Tries to expire the oldest |max_visits| visits from history that are older
   // than |time_threshold|. The return value indicates if we think there might
   // be more history to expire with the current time threshold (it does not