Drop [TypeChecking=Nullable]; deduce from attribute type instead

Source/bindings/tests/idls/TestInterface.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 21 matching lines @@
 // Also used as a target by TestObject
 
 [
     ActiveDOMObject,
     Conditional=CONDITION,
     Custom=LegacyCallAsFunction|ToV8,
     DoNotCheckConstants,
     ImplementedAs=TestInterfaceImplementation,
     RuntimeEnabled=FeatureName,
     SetWrapperReferenceTo(TestInterface referencedName),
-    TypeChecking=Interface|Nullable|Unrestricted,
+    TypeChecking=Interface|Unrestricted,
 ] interface TestInterface : TestInterfaceEmpty {
     // members needed to test [ImplementedAs], as this affect attribute
     // configuration and method configuration, and [TypeChecking]
     // constants also needed for [DoNotCheckConstants]
     const unsigned long UNSIGNED_LONG = 0;
     [Reflect=CONST_CPP] const short CONST_JAVASCRIPT = 1;
 
     attribute TestInterface testInterfaceAttribute; // Self-referential interface type with [ImplementedAs]
     attribute TestInterfaceConstructor testInterfaceConstructorAttribute;
     attribute double doubleAttribute;
@@ skipping 16 matching lines @@
     getter DOMString (DOMString name);
     setter DOMString (DOMString name, DOMString value);
     deleter boolean (DOMString name);
 
     [NotEnumerable] stringifier;
 };
 
 TestInterface implements TestImplements;
 // TestInterface implements TestImplements2; // at implement*ed* interface
 TestInterface implements TestImplements3;