Make DataTransferItem.getAsString() argument mandatory and nullable

Source/bindings/tests/idls/TestObject.idl

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary formstrArg, 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 130 matching lines @@
     void    methodWithNonOptionalArgAndOptionalArg(long nonOpt, optional long opt);
     void    methodWithNonOptionalArgAndTwoOptionalArgs(long nonOpt, optional long opt1, optional long opt2);
     void    methodWithOptionalString(optional DOMString str);
     void    methodWithOptionalStringIsUndefined([Default=Undefined] optional DOMString str);
     void    methodWithOptionalStringIsNullString([Default=NullString] optional DOMString str);
 
     // Callback parameters
     void    methodWithCallbackArg(TestCallback callback);
     void    methodWithNonCallbackArgAndCallbackArg(long nonCallback, TestCallback callback);
     void    methodWithCallbackAndOptionalArg(optional TestCallback callback);
+    void    methodWithNullableCallbackArg(TestCallback? callback);
     // static methods with Callback parameter
     static void    staticMethodWithCallbackAndOptionalArg(optional TestCallback callback);
     static void    staticMethodWithCallbackArg(TestCallback callback);
 
     // 'EnforceRange' extended attribute
     void    methodWithEnforceRangeInt8([EnforceRange] byte value);
     void    methodWithEnforceRangeUInt8([EnforceRange] octet value);
     void    methodWithEnforceRangeInt32([EnforceRange] long value);
     void    methodWithEnforceRangeUInt32([EnforceRange] unsigned long value);
     void    methodWithEnforceRangeInt64([EnforceRange] long long value);
@@ skipping 157 matching lines @@
 
     [DeprecateAs=Method] void deprecatedMethod();
     [DeprecateAs=StaticMethod] static void deprecatedStaticMethod();
 
     [DeprecateAs=Constant] const unsigned short DEPRECATED_CONSTANT = 1;
 };
 
 // The following comment should not generate any code
 // TestObject implements TestImplements;