[Background Sync] Align exposed API with spec

third_party/WebKit/LayoutTests/http/tests/background_sync/resources/test-helpers.js

-// Returns a promise which resolves when all registrations have been cleared
-// from the given background sync manager.
-function clear_registered_syncs(sync_manager) {
-  return sync_manager.getRegistrations().then(function(registrations) {
-    return Promise.all(
-      registrations.map(registration => registration.unregister()));
-  });
-}
-
-// Clears all background sync registrations from the sync manager.
-function clear_all_syncs(serviceworker_registration) {
-  return clear_registered_syncs(serviceworker_registration.sync);
-}