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

Source/core/rendering/RenderListMarker.cpp

Index: Source/core/rendering/RenderListMarker.cpp
diff --git a/Source/core/rendering/RenderListMarker.cpp b/Source/core/rendering/RenderListMarker.cpp
index 930b587be7ac71bdc2cb5119e95ac4e82cb16929..827abb9321b0160022adba732ae0c5f5ac15513a 100644
--- a/Source/core/rendering/RenderListMarker.cpp
+++ b/Source/core/rendering/RenderListMarker.cpp
@@ -1070,9 +1070,9 @@ RenderListMarker::~RenderListMarker()
 
 RenderListMarker* RenderListMarker::createAnonymous(RenderListItem* item)
 {
-    Document* document = item->document();
+    Document& document = item->document();
     RenderListMarker* renderer = new RenderListMarker(item);
-    renderer->setDocumentForAnonymous(document);
+    renderer->setDocumentForAnonymous(&document);
     return renderer;
 }