Oilpan: improve ScrollableArea handling.

Source/core/frame/PinchViewport.h

Index: Source/core/frame/PinchViewport.h
diff --git a/Source/core/frame/PinchViewport.h b/Source/core/frame/PinchViewport.h
index 69871ffa41720f1148a973251aed45a20dcd98b6..e2c72812e9cd693ac044a9908658c303e168e50f 100644
--- a/Source/core/frame/PinchViewport.h
+++ b/Source/core/frame/PinchViewport.h
@@ -62,14 +62,17 @@ class LocalFrame;
 // offset is set through the GraphicsLayer <-> CC sync mechanisms. Its contents is the page's
 // main FrameView, which corresponds to the outer viewport. The inner viewport is always contained
 // in the outer viewport and can pan within it.
-class CORE_EXPORT PinchViewport final : public GraphicsLayerClient, public ScrollableArea {
+class CORE_EXPORT PinchViewport final : public NoBaseWillBeGarbageCollectedFinalized<PinchViewport>, public GraphicsLayerClient, public ScrollableArea {
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(PinchViewport);
 public:
-    static PassOwnPtr<PinchViewport> create(FrameHost& host)
+    static PassOwnPtrWillBeRawPtr<PinchViewport> create(FrameHost& host)
     {
-        return adoptPtr(new PinchViewport(host));
+        return adoptPtrWillBeNoop(new PinchViewport(host));
     }
     virtual ~PinchViewport();
 
+    DECLARE_VIRTUAL_TRACE();
+
     void attachToLayerTree(GraphicsLayer*, GraphicsLayerFactory*);
     GraphicsLayer* rootGraphicsLayer()
     {
@@ -203,13 +206,11 @@ private:
 
     FrameHost& frameHost() const
     {
-        return m_frameHost;
+        ASSERT(m_frameHost);
+        return *m_frameHost;
     }
 
-    // TODO(Oilpan): this back reference is safe, but not ideal.
-    // Turning it into a traced Member<> would require moving
-    // ScrollableArea to the heap.
-    FrameHost& m_frameHost;
+    RawPtrWillBeMember<FrameHost> m_frameHost;
     OwnPtr<GraphicsLayer> m_rootTransformLayer;
     OwnPtr<GraphicsLayer> m_innerViewportContainerLayer;
     OwnPtr<GraphicsLayer> m_overscrollElasticityLayer;