NTPSnippetsService: Garbage collect orphaned images at startup.

components/ntp_snippets/remote/ntp_snippets_service.h

Index: components/ntp_snippets/remote/ntp_snippets_service.h
diff --git a/components/ntp_snippets/remote/ntp_snippets_service.h b/components/ntp_snippets/remote/ntp_snippets_service.h
index ad3ad2bdeb94f541985ab19249ef8b90005d2e68..dad3c3617a5cee00abc288d88a6fca5b986fe8ec 100644
--- a/components/ntp_snippets/remote/ntp_snippets_service.h
+++ b/components/ntp_snippets/remote/ntp_snippets_service.h
@@ -235,9 +235,11 @@ class NTPSnippetsService final : public ContentSuggestionsProvider,
   // Removes expired dismissed snippets from the service and the database.
   void ClearExpiredDismissedSnippets();
 
-  // Removes images from the DB that do not have any corresponding snippet
-  // (neither in the current set, nor in the archived set).
-  void ClearOrphanedImages();
+  // Removes images from the DB that do not have any corresponding snippet in
+  // the provided set. Needs to iterate the whole snippet database -- so do it
+  // often enough to keep it small but not too often as it still iterates over
+  // the file system.
+  void ClearOrphanedImages(const NTPSnippet::PtrVector& snippets);
 
   // Clears all stored snippets and updates the observer.
   void NukeAllSnippets();