Oilpan: have DOMException be on the heap by default.

Source/core/streams/ReadableStreamReader.h

 // Copyright 2015 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.
 
 #ifndef ReadableStreamReader_h
 #define ReadableStreamReader_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptPromiseProperty.h"
 #include "bindings/core/v8/ScriptValue.h"
@@ skipping 27 matching lines @@
     ScriptPromise read(ScriptState*);
     void releaseLock(ExceptionState&);
     void releaseLock();
 
     bool hasPendingActivity() const override;
     void stop() override;
 
     DECLARE_TRACE();
 
 private:
-    using ClosedPromise = ScriptPromiseProperty<Member<ReadableStreamReader>, ToV8UndefinedGenerator, RefPtrWillBeMember<DOMException>>;
+    using ClosedPromise = ScriptPromiseProperty<Member<ReadableStreamReader>, ToV8UndefinedGenerator, Member<DOMException>>;
 
     const Member<ReadableStream> m_stream;
     ReadableStream::State m_stateAfterRelease;
     Member<ClosedPromise> m_closed;
 };
 
 } // namespace blink
 
 #endif // ReadableStreamReader_h