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

Source/core/css/StyleSheetList.h

Index: Source/core/css/StyleSheetList.h
diff --git a/Source/core/css/StyleSheetList.h b/Source/core/css/StyleSheetList.h
index c9c15404729e68479144745f625b23fecf873654..d4e8108464e7e9b92f6809171d9b8b118adb0d21 100644
--- a/Source/core/css/StyleSheetList.h
+++ b/Source/core/css/StyleSheetList.h
@@ -43,7 +43,8 @@ public:
 
     HTMLStyleElement* getNamedItem(const AtomicString&) const;
 
-    Document* document() { return m_treeScope->documentScope(); }
+    // FIXME: Should return a reference.
+    Document* document() { return &m_treeScope->documentScope(); }
 
     void detachFromDocument();
     CSSStyleSheet* anonymousNamedGetter(const AtomicString&);