[ServiceWorker] Add missing method to Cache.idl.

The string version of Add() was unfortunately omitted in an earlier CL.

R=falken@chromium.org,jkarlin@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/458853002

git-svn-id: svn://svn.chromium.org/blink/trunk@179946 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/serviceworkers/Cache.idl

@@ -33,6 +33,7 @@
 
   // [CallWith=ScriptState] Promise add((Request or ScalarValueString) request);
   [CallWith=ScriptState] Promise add(Request request);
+  [CallWith=ScriptState] Promise add(ScalarValueString request);
 
   // FIXME: The lack of union type support together with the sequence mean we can't use overrides here, instead
   // bind a generic type.