Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

Source/core/plugins/PluginOcclusionSupport.cpp

Index: Source/core/plugins/PluginOcclusionSupport.cpp
diff --git a/Source/core/plugins/PluginOcclusionSupport.cpp b/Source/core/plugins/PluginOcclusionSupport.cpp
index d6379966c33548188dcba20ffe3ce2ea778872c9..f5541414c6b74cf09e27e976513709ebe8b8ecf4 100644
--- a/Source/core/plugins/PluginOcclusionSupport.cpp
+++ b/Source/core/plugins/PluginOcclusionSupport.cpp
@@ -193,7 +193,7 @@ void getPluginOcclusions(Element* element, Widget* parentWidget, const IntRect&
     // as being in the top layer.
     const Element* ancestor = topLayerAncestor(element);
     Document* document = parentFrameView->frame().document();
-    const Vector<RefPtr<Element> >& elements = document->topLayerElements();
+    const WillBeHeapVector<RefPtrWillBeMember<Element> >& elements = document->topLayerElements();
     size_t start = ancestor ? elements.find(ancestor) + 1 : 0;
     for (size_t i = start; i < elements.size(); ++i)
         addTreeToOcclusions(elements[i]->renderer(), frameRect, occlusions);