PaymentHandler: Implement PaymentInstruments.keys().

content/browser/service_worker/service_worker_database.h

Index: content/browser/service_worker/service_worker_database.h
diff --git a/content/browser/service_worker/service_worker_database.h b/content/browser/service_worker/service_worker_database.h
index a4f208d32d28f6ebd159f160977859ed0fac032f..8eadc405fc4d598e99f45f752990012999371159 100644
--- a/content/browser/service_worker/service_worker_database.h
+++ b/content/browser/service_worker/service_worker_database.h
@@ -198,6 +198,13 @@ class CONTENT_EXPORT ServiceWorkerDatabase {
                       const std::vector<std::string>& user_data_names,
                       std::vector<std::string>* user_data_values);
 
+  // Reads user data for |registration_id| and |key_prefix| from the database.
+  // Returns OK only if keys matched to |key_prefix| are found; otherwise
+  // NOT_FOUND, and |user_data_values| will be empty.
+  Status ReadUserDataByKeyPrefix(int64_t registration_id,
+                                 const std::string key_prefix,
+                                 std::vector<std::string>* user_data_values);
+
   // Writes |name_value_pairs| into the database. Returns NOT_FOUND if the
   // registration specified by |registration_id| does not exist in the database.
   Status WriteUserData(