Extended the ProtoDatabase to provide LoadKeys() functionality.

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 43ed9960ffe7d0868b4f6d230768ba928271cb94..f4e37427bab31a433dcc62a60a67e2a508afc093 100644
--- a/components/ntp_snippets/remote/ntp_snippets_service.h
+++ b/components/ntp_snippets/remote/ntp_snippets_service.h
@@ -231,9 +231,11 @@ 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();