StyleSheetList::document() should return null after detaching the StyleSheetList from document.

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 9750565e646be405ebd403569ceb4e3fc8dbc28d..2e371195692ac0581f70ca11f63c67f2d856c3c4 100644
--- a/Source/core/css/StyleSheetList.h
+++ b/Source/core/css/StyleSheetList.h
@@ -43,8 +43,7 @@ public:
 
     HTMLStyleElement* getNamedItem(const AtomicString&) const;
 
-    // FIXME: Should return a reference.
-    Document* document() { return &m_treeScope->document(); }
+    Document* document() { return m_treeScope ? &m_treeScope->document() : 0; }
 
     void detachFromDocument();
     CSSStyleSheet* anonymousNamedGetter(const AtomicString&);