Oilpan: Build fix after r192015

Source/web/RotationViewportAnchor.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 RotationViewportAnchor_h
 #define RotationViewportAnchor_h
 
 #include "platform/geometry/FloatSize.h"
 #include "platform/geometry/IntPoint.h"
 #include "platform/geometry/IntRect.h"
@@ skipping 38 matching lines @@
     float m_oldPageScaleFactor;
     float m_oldMinimumPageScaleFactor;
 
     // Inner viewport origin in the reference frame of the document in CSS pixels
     FloatPoint m_pinchViewportInDocument;
 
     // Inner viewport origin in the reference frame of the outer viewport
     // normalized to the outer viewport size.
     FloatSize m_normalizedPinchViewportOffset;
 
-    RefPtrWillBeMember<Node> m_anchorNode;
+    RefPtrWillBePersistent<Node> m_anchorNode;
     LayoutRect m_anchorNodeBounds;
 
     FloatSize m_anchorInInnerViewCoords;
     FloatSize m_anchorInNodeCoords;
 
     PageScaleConstraintsSet& m_pageScaleConstraintsSet;
 };
 
 } // namespace blink
 
 #endif