IDL: Support nullable union types

Source/bindings/tests/idls/core/TestObject.idl

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 139 matching lines @@
     attribute TestInterface? testInterfaceOrNullAttribute;
     // Enumerations
     attribute TestEnum testEnumAttribute;
     // Static attributes
     static attribute DOMString staticStringAttribute;
     static attribute long staticLongAttribute;
     // Exceptional type
     attribute EventHandler eventHandlerAttribute;
     // Union types
     attribute (double or DOMString) doubleOrStringAttribute;
+    attribute (double or DOMString)? doubleOrStringOrNullAttribute;
 
     // Extended attributes
     [LogActivity, LogAllWorlds] attribute long activityLoggingAccessForAllWorldsLongAttribute;
     [LogActivity=GetterOnly, LogAllWorlds] attribute long activityLoggingGetterForAllWorldsLongAttribute;
     [LogActivity=SetterOnly, LogAllWorlds] attribute long activityLoggingSetterForAllWorldsLongAttribute;
     [CachedAttribute=isValueDirty] attribute any cachedAttributeAnyAttribute;
     [CachedAttribute=isArrayDirty] attribute DOMString[] cachedArrayAttribute;
     [CachedAttribute=isStringDirty] attribute DOMString? cachedStringOrNoneAttribute;
     [CallWith=ExecutionContext] attribute any callWithExecutionContextAnyAttribute;
     [CallWith=ScriptState] attribute any callWithScriptStateAnyAttribute;
@@ skipping 387 matching lines @@
     [ImplementedInPrivateScript] readonly attribute short readonlyShortAttribute;
     [ImplementedInPrivateScript] attribute short shortAttribute;
     [ImplementedInPrivateScript] attribute DOMString stringAttribute;
     [ImplementedInPrivateScript] attribute Node nodeAttribute;
     [OnlyExposedToPrivateScript] short methodImplementedInCPPForPrivateScriptOnly(short value1, short value2);
     [OnlyExposedToPrivateScript] attribute DOMString attributeImplementedInCPPForPrivateScriptOnly;
     [ImplementedInPrivateScript, OnlyExposedToPrivateScript] short methodForPrivateScriptOnly(short value1, short value2);
     [ImplementedInPrivateScript, OnlyExposedToPrivateScript] attribute DOMString attributeForPrivateScriptOnly;
     [ImplementedInPrivateScript] attribute TestEnum enumForPrivateScript;
 };