Have bindings layer assume and insist that all interface types are GCed.

third_party/WebKit/Source/core/testing/UnionTypesTest.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
-    GarbageCollected
 ] interface UnionTypesTest {
     attribute (double or DOMString or DOMString[]) doubleOrStringOrStringArrayAttribute;
 
     DOMString doubleOrStringArg((double or DOMString) arg);
     DOMString doubleOrInternalEnumArg((double or InternalEnum) arg);
 
     [ImplementedAs=doubleOrStringArg] DOMString doubleOrStringDefaultDoubleArg(optional (double or DOMString) arg = 3.14);
     [ImplementedAs=doubleOrStringArg] DOMString doubleOrStringDefaultStringArg(optional (double or DOMString) arg = "foo");
     [ImplementedAs=doubleOrStringArg] DOMString doubleOrStringDefaultNullArg(optional (double or DOMString)? arg = null);
 
     DOMString doubleOrStringArrayArg((double or DOMString)[] arg);
     DOMString doubleOrStringSequenceArg(sequence<(double or DOMString)> arg);
 
     DOMString nodeListOrElementArg((NodeList or Element) arg);
     DOMString nodeListOrElementOrNullArg((NodeList or Element)? arg);
 
     DOMString doubleOrStringOrStringArrayArg((double or DOMString or DOMString[]) arg);
     DOMString doubleOrStringOrStringSequenceArg((double or DOMString or sequence<DOMString>) arg);
     [ImplementedAs=doubleOrStringOrStringArrayArg] DOMString doubleOrStringOrStringArrayNullableArg((double or DOMString or DOMString[])? arg);
     [ImplementedAs=doubleOrStringOrStringSequenceArg] DOMString doubleOrStringOrStringSequenceNullableArg((double or DOMString or sequence<DOMString>)? arg);
 };