Oilpan: fix build after r368814.

third_party/WebKit/Source/core/dom/IntersectionObserverController.cpp

 // Copyright 2016 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.
 
 #include "core/dom/IntersectionObserverController.h"
 
 #include "core/dom/Document.h"
 
 namespace blink {
 
@@ skipping 67 matching lines @@
     HeapVector<Member<IntersectionObserver>> toRemove;
     for (auto& observer : m_trackedIntersectionObservers) {
         if (observer->root() == &root)
             toRemove.append(observer);
     }
     m_trackedIntersectionObservers.removeAll(toRemove);
 }
 
 DEFINE_TRACE(IntersectionObserverController)
 {
-    ActiveDOMObject::trace(visitor);
     visitor->trace(m_trackedIntersectionObservers);
     visitor->trace(m_pendingIntersectionObservers);
+    ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink