IDL: Use Nullable for union type return value

Source/bindings/tests/idls/TestTypedefs.idl

Index: Source/bindings/tests/idls/TestTypedefs.idl
diff --git a/Source/bindings/tests/idls/TestTypedefs.idl b/Source/bindings/tests/idls/TestTypedefs.idl
index 373a875dc13e37a71de36e7dfff556d6e2beb143..996f1c11b15cdc1ff702ed1fcb5a8999fe741a1c 100644
--- a/Source/bindings/tests/idls/TestTypedefs.idl
+++ b/Source/bindings/tests/idls/TestTypedefs.idl
@@ -41,6 +41,7 @@ typedef DOMString[]                ArrayOfStrings;
 typedef TestCallbackInterface      TestCallbackInterfaceType;
 typedef TestInterfaceConstructor   T;
 typedef (TestInterface or TestInterfaceEmpty) TestInterfaceOrTestInterfaceEmpty;
+typedef (DOMString or double) DOMStringOrDouble;
 
 [
     Constructor(String stringArg),
@@ -55,6 +56,7 @@ typedef (TestInterface or TestInterfaceEmpty) TestInterfaceOrTestInterfaceEmpty;
 
     ULongLong uLongLongMethodTestInterfaceEmptyTypeSequenceArg(sequence<TestInterfaceEmptyType> testInterfaceEmptyTypeSequenceArg);
     TestInterfaceOrTestInterfaceEmpty testInterfaceOrTestInterfaceEmptyMethod();
+    DOMStringOrDouble domStringOrDoubleMethod();
 
     ArrayOfStrings arrayOfStringsMethodArrayOfStringsArg(ArrayOfStrings arrayOfStringsArg);
     String[] stringArrayMethodStringArrayArg(String[] stringArrayArg);