Oilpan: improve ScrollableArea handling.

Source/core/frame/PinchViewport.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 50 matching lines @@
 using blink::WebLayerTreeView;
 using blink::WebScrollbar;
 using blink::WebScrollbarLayer;
 using blink::FrameHost;
 using blink::GraphicsLayer;
 using blink::GraphicsLayerFactory;
 
 namespace blink {
 
 PinchViewport::PinchViewport(FrameHost& owner)
-    : m_frameHost(owner)
+    : m_frameHost(&owner)
     , m_scale(1)
     , m_topControlsAdjustment(0)
 {
     reset();
 }
 
 PinchViewport::~PinchViewport()
 {
 }
 
+DEFINE_TRACE(PinchViewport)
+{
+    visitor->trace(m_frameHost);
+    ScrollableArea::trace(visitor);
+}
+
 void PinchViewport::setSize(const IntSize& size)
 {
     // When the main frame is remote, we won't have an associated frame.
     if (!mainFrame())
         return;
 
     if (m_size == size)
         return;
 
     bool autosizerNeedsUpdating =
@@ skipping 561 matching lines @@
     } else if (graphicsLayer == m_rootTransformLayer) {
         name =  "Root Transform Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink