Oilpan: improve ScrollableArea handling.

Source/core/paint/DeprecatedPaintLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 682 matching lines @@
 
     // Once computed, indicates all that a layer needs to become composited using the CompositingReasons enum bitfield.
     CompositingReasons m_compositingReasons;
 
     DescendantDependentCompositingInputs m_descendantDependentCompositingInputs;
     AncestorDependentCompositingInputs m_ancestorDependentCompositingInputs;
 
     IntRect m_blockSelectionGapsBounds;
 
     OwnPtr<CompositedDeprecatedPaintLayerMapping> m_compositedDeprecatedPaintLayerMapping;
-    OwnPtr<DeprecatedPaintLayerScrollableArea> m_scrollableArea;
+    OwnPtrWillBePersistent<DeprecatedPaintLayerScrollableArea> m_scrollableArea;
 
     CompositedDeprecatedPaintLayerMapping* m_groupedMapping;
 
     DeprecatedPaintLayerClipper m_clipper; // FIXME: Lazily allocate?
     OwnPtr<DeprecatedPaintLayerStackingNode> m_stackingNode;
     OwnPtr<DeprecatedPaintLayerReflectionInfo> m_reflectionInfo;
 
     LayoutSize m_subpixelAccumulation; // The accumulated subpixel offset of a composited layer's composited bounds compared to absolute coordinates.
 };
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const blink::DeprecatedPaintLayer*);
 void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif // Layer_h