Add ClearHistory() to ContentSuggestionsService and its providers

components/ntp_snippets/content_suggestions_service.h

Index: components/ntp_snippets/content_suggestions_service.h
diff --git a/components/ntp_snippets/content_suggestions_service.h b/components/ntp_snippets/content_suggestions_service.h
index 4f36237e62bdf74c233655eb6a26fdb9d6baf6a8..3e4318f4fa1dd591767fd151f45e8c8029b9ef1a 100644
--- a/components/ntp_snippets/content_suggestions_service.h
+++ b/components/ntp_snippets/content_suggestions_service.h
@@ -120,6 +120,9 @@ class ContentSuggestionsService : public KeyedService,
   // called only once per provider.
   void RegisterProvider(std::unique_ptr<ContentSuggestionsProvider> provider);
 
+  // Removes history from all providers, for example dismissed suggestions.
+  void ClearHistory();
+
   // Removes all suggestions from all caches or internal stores in all
   // providers. See |ClearCachedSuggestions|.
   void ClearAllCachedSuggestions();