toImplArguments should return HeapVector for 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 358 matching lines @@
     // Union types
     (TestInterfaceGarbageCollected or DOMString) testInterfaceGarbageCollectedOrDOMStringMethod();
     (TestInterfaceWillBeGarbageCollected or TestDictionary) testInterfaceWillBeGarbageCollectedOrTestDictionaryMethod();
     (boolean or DOMString or unrestricted double) booleanOrDOMStringOrUnrestrictedDoubleMethod();
     (TestInterface or long) testInterfaceOrLongMethod();
     void voidMethodDoubleOrDOMStringArg((double or DOMString) arg);
     void voidMethodDoubleOrDOMStringOrNullArg((double or DOMString)? arg);
     void voidMethodDoubleOrNullOrDOMStringArg((double? or DOMString) arg);
     void voidMethodDOMStringOrArrayBufferOrArrayBufferViewArg((DOMString or ArrayBuffer or ArrayBufferView) arg);
     void voidMethodArrayBufferOrArrayBufferViewOrDictionaryArg((ArrayBuffer or ArrayBufferView or Dictionary) arg);
+    // Array of Union types
+    void voidMethodArrayOfDoubleOrDOMStringArg((double or DOMString)... arg);
     // Currently only used on interface type arguments
     void voidMethodTestInterfaceEmptyOrNullArg(TestInterfaceEmpty? nullableTestInterfaceEmptyArg);
     // Callback interface types
     void voidMethodTestCallbackInterfaceArg(TestCallbackInterface testCallbackInterfaceArg);
     void voidMethodOptionalTestCallbackInterfaceArg(optional TestCallbackInterface optionalTestCallbackInterfaceArg);
     void voidMethodTestCallbackInterfaceOrNullArg(TestCallbackInterface? testCallbackInterfaceArg);
     // Enumerations
     TestEnum testEnumMethod();
     void voidMethodTestEnumArg(TestEnum testEnumTypeArg);
     // Exceptional types
@@ skipping 216 matching lines @@
     [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;
 
     maplike<long, DOMStringOrDouble>;
 };