Have Node::document() return a reference instead of a pointer

Source/core/rendering/RenderLayerBacking.cpp

Index: Source/core/rendering/RenderLayerBacking.cpp
diff --git a/Source/core/rendering/RenderLayerBacking.cpp b/Source/core/rendering/RenderLayerBacking.cpp
index 991d529755e2b4362627f51818ffca24254383f2..264c2f025e73e53c84464a9ce5c81a702f056e7e 100644
--- a/Source/core/rendering/RenderLayerBacking.cpp
+++ b/Source/core/rendering/RenderLayerBacking.cpp
@@ -1219,7 +1219,7 @@ bool RenderLayerBacking::isSimpleContainerCompositingLayer() const
 
     if (renderObject->node() && renderObject->node()->isDocumentNode()) {
         // Look to see if the root object has a non-simple background
-        RenderObject* rootObject = renderObject->document()->documentElement() ? renderObject->document()->documentElement()->renderer() : 0;
+        RenderObject* rootObject = renderObject->document().documentElement() ? renderObject->document().documentElement()->renderer() : 0;
         if (!rootObject)
             return false;
 
@@ -1231,7 +1231,7 @@ bool RenderLayerBacking::isSimpleContainerCompositingLayer() const
             return false;
 
         // Now look at the body's renderer.
-        HTMLElement* body = renderObject->document()->body();
+        HTMLElement* body = renderObject->document().body();
         RenderObject* bodyObject = (body && body->hasLocalName(bodyTag)) ? body->renderer() : 0;
         if (!bodyObject)
             return false;