Oilpan: have DOMException be on the heap by default.

Source/modules/background_sync/SyncError.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 SyncError_h
 #define SyncError_h
 
-#include "core/dom/DOMException.h"
 #include "platform/heap/Handle.h"
 #include "public/platform/modules/background_sync/WebSyncError.h"
 
 namespace blink {
 
+class DOMException;
 class ScriptPromiseResolver;
 
 class SyncError {
     WTF_MAKE_NONCOPYABLE(SyncError);
 public:
     // For CallbackPromiseAdapter.
     using WebType = WebSyncError;
-    static PassRefPtrWillBeRawPtr<DOMException> take(ScriptPromiseResolver*, WebType*);
+    static DOMException* take(ScriptPromiseResolver*, WebType*);
     static void dispose(WebType*);
 
 private:
     SyncError() = delete;
 };
 
 } // namespace blink
 
 #endif // SyncError_h