Have StyleSheetCollections constructor take a Document reference in argument

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 5aadeae401b461b158d7aacd1f310c7bdfa7db53..d3e0e8bdb260bcefc625ee3eb8b69a639893cb2b 100644
--- a/Source/core/dom/Node.h
+++ b/Source/core/dom/Node.h
@@ -492,6 +492,8 @@ public:
         return *documentInternal();
     }
 
+    // Can never return 0.
+    // FIXME: This should return a reference.
     TreeScope* treeScope() const { return m_treeScope; }
 
     // Returns true if this node is associated with a document and is in its associated document's