bindings: Supports [LenientThis] extended attributes.

Source/bindings/tests/idls/core/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 96 matching lines @@
     [Exposed=(Window,ServiceWorker)] void windowAndServiceWorkerExposedMethod();
 
     void voidMethodPartialOverload();
     void voidMethodPartialOverload(double doubleArg);
     static void staticVoidMethodPartialOverload();
 
     Promise promiseMethodPartialOverload();
     Promise promiseMethodPartialOverload(Window window);
     static Promise staticPromiseMethodPartialOverload();
 
+    [LenientThis] attribute any lenientThisAttribute;
+
     [LegacyInterfaceTypeChecking] void legacyInterfaceTypeCheckingMethod(TestInterfaceEmpty testInterfaceEmptyArg);
 };
 
 TestInterface implements TestImplements;
 // TestInterface implements TestImplements2; // at implement*ed* interface
 TestInterface implements TestImplements3;