Use IDL argument default values in modules/indexeddb/

Source/modules/indexeddb/IDBDatabase.idl

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 18 matching lines @@
 [
     ActiveDOMObject,
     GarbageCollected,
 ] interface IDBDatabase : EventTarget {
     readonly attribute DOMString name;
     [CallWith=ScriptState] readonly attribute any version;
     readonly attribute DOMStringList objectStoreNames;
     [RaisesException] IDBObjectStore createObjectStore(DOMString name, optional Dictionary options);
     [RaisesException] void deleteObjectStore(DOMString name);
     // FIXME: should be union type http://crbug.com/240176
-    [CallWith=ExecutionContext, RaisesException] IDBTransaction transaction(DOMString storeName, optional DOMString mode = null);
-    [CallWith=ExecutionContext, RaisesException] IDBTransaction transaction(sequence<DOMString> storeNames, optional DOMString mode = null);
-    [CallWith=ExecutionContext, RaisesException] IDBTransaction transaction(DOMStringList storeNames, optional DOMString mode = null);
+    [CallWith=ExecutionContext, RaisesException] IDBTransaction transaction(DOMString storeName, optional DOMString mode = "readonly");
+    [CallWith=ExecutionContext, RaisesException] IDBTransaction transaction(sequence<DOMString> storeNames, optional DOMString mode = "readonly");
+    [CallWith=ExecutionContext, RaisesException] IDBTransaction transaction(DOMStringList storeNames, optional DOMString mode = "readonly");
     void close();
     attribute EventHandler onabort;
     attribute EventHandler onclose;
     attribute EventHandler onerror;
     attribute EventHandler onversionchange;
 };