Have Document::topDocument() return a reference

Source/core/html/HTMLDialogElement.cpp

Index: Source/core/html/HTMLDialogElement.cpp
diff --git a/Source/core/html/HTMLDialogElement.cpp b/Source/core/html/HTMLDialogElement.cpp
index 566624439aae28f1e0fa8e5f313187049cda5985..be7af1bd8b6d156452628c8da50da61558c69405 100644
--- a/Source/core/html/HTMLDialogElement.cpp
+++ b/Source/core/html/HTMLDialogElement.cpp
@@ -83,10 +83,10 @@ static void inertSubtreesChanged(Document& document)
     // tree can change inertness which means they must be added or removed from
     // the tree. The most foolproof way is to clear the entire tree and rebuild
     // it, though a more clever way is probably possible.
-    Document* topDocument = document.topDocument();
-    topDocument->clearAXObjectCache();
-    if (AXObjectCache* cache = topDocument->axObjectCache())
-        cache->childrenChanged(cache->getOrCreate(topDocument));
+    Document& topDocument = document.topDocument();
+    topDocument.clearAXObjectCache();
+    if (AXObjectCache* cache = topDocument.axObjectCache())
+        cache->childrenChanged(cache->getOrCreate(&topDocument));
 }
 
 HTMLDialogElement::HTMLDialogElement(Document& document)