Make TreeScope::documentScope() return a reference and ensure m_documentScope is never NULL

Source/core/dom/Node.h

Index: Source/core/dom/Node.h
diff --git a/Source/core/dom/Node.h b/Source/core/dom/Node.h
index 008771001f0a298e8253143fc579702c87ce7696..e352ef37c9c5bfc6a456e8151f932eadf801a254 100644
--- a/Source/core/dom/Node.h
+++ b/Source/core/dom/Node.h
@@ -457,8 +457,7 @@ public:
     // Returns the document associated with this node. A Document node returns itself.
     Document& document() const
     {
-        ASSERT(treeScope().documentScope());
-        return *treeScope().documentScope();
+        return treeScope().documentScope();
     }
 
     TreeScope& treeScope() const