Have Node::document() return a reference instead of a pointer

Source/core/html/HTMLMapElement.cpp

Index: Source/core/html/HTMLMapElement.cpp
diff --git a/Source/core/html/HTMLMapElement.cpp b/Source/core/html/HTMLMapElement.cpp
index ee6b009096e36bd3e764278334b61ed1c7fe6bb3..b3eea224b214322e1df514ba630c53bcdb70cb79 100644
--- a/Source/core/html/HTMLMapElement.cpp
+++ b/Source/core/html/HTMLMapElement.cpp
@@ -81,7 +81,7 @@ bool HTMLMapElement::mapMouseEvent(LayoutPoint location, const LayoutSize& size,
 
 HTMLImageElement* HTMLMapElement::imageElement()
 {
-    RefPtr<HTMLCollection> images = document()->images();
+    RefPtr<HTMLCollection> images = document().images();
     for (unsigned i = 0; Node* curr = images->item(i); i++) {
         if (!curr->hasTagName(imgTag))
             continue;
@@ -106,7 +106,7 @@ void HTMLMapElement::parseAttribute(const QualifiedName& name, const AtomicStrin
         if (isIdAttributeName(name)) {
             // Call base class so that hasID bit gets set.
             HTMLElement::parseAttribute(name, value);
-            if (document()->isHTMLDocument())
+            if (document().isHTMLDocument())
                 return;
         }
         if (inDocument())
@@ -114,7 +114,7 @@ void HTMLMapElement::parseAttribute(const QualifiedName& name, const AtomicStrin
         String mapName = value;
         if (mapName[0] == '#')
             mapName = mapName.substring(1);
-        m_name = document()->isHTMLDocument() ? mapName.lower() : mapName;
+        m_name = document().isHTMLDocument() ? mapName.lower() : mapName;
         if (inDocument())
             treeScope()->addImageMap(this);