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

Source/core/dom/Element.h

Index: Source/core/dom/Element.h
diff --git a/Source/core/dom/Element.h b/Source/core/dom/Element.h
index 1b35d586547e802b1d3fd71aeff40bb827d7a799..5b060c5926fb0fa12fa19870fd405713abdd55fd 100644
--- a/Source/core/dom/Element.h
+++ b/Source/core/dom/Element.h
@@ -884,12 +884,12 @@ inline bool Element::isIdAttributeName(const QualifiedName& attributeName) const
     // with a non-null namespace, because it will return false, a false negative, if the prefixes
     // don't match but the local name and namespace both do. However, since this has been like this
     // for a while and the code paths may be hot, we'll have to measure performance if we fix it.
-    return attributeName == document()->idAttributeName();
+    return attributeName == document().idAttributeName();
 }
 
 inline const AtomicString& Element::getIdAttribute() const
 {
-    return hasID() ? fastGetAttribute(document()->idAttributeName()) : nullAtom;
+    return hasID() ? fastGetAttribute(document().idAttributeName()) : nullAtom;
 }
 
 inline const AtomicString& Element::getNameAttribute() const
@@ -908,7 +908,7 @@ inline const AtomicString& Element::getClassAttribute() const
 
 inline void Element::setIdAttribute(const AtomicString& value)
 {
-    setAttribute(document()->idAttributeName(), value);
+    setAttribute(document().idAttributeName(), value);
 }
 
 inline const SpaceSplitString& Element::classNames() const