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

Source/core/css/resolver/ScopedStyleResolver.cpp

Index: Source/core/css/resolver/ScopedStyleResolver.cpp
diff --git a/Source/core/css/resolver/ScopedStyleResolver.cpp b/Source/core/css/resolver/ScopedStyleResolver.cpp
index 261d45818bf572f1b9e2a3d52c0fb20d4156c60a..53c76a3b7236141a25b8844b33c0e6340f1f1326 100644
--- a/Source/core/css/resolver/ScopedStyleResolver.cpp
+++ b/Source/core/css/resolver/ScopedStyleResolver.cpp
@@ -143,12 +143,12 @@ void ScopedStyleTree::collectScopedResolversForHostedShadowTrees(const Element*
 
 void ScopedStyleTree::resolveScopedKeyframesRules(const Element* element, Vector<ScopedStyleResolver*, 8>& resolvers)
 {
-    Document* document = element->document();
+    Document& document = element->document();
     TreeScope* treeScope = element->treeScope();
     bool applyAuthorStyles = treeScope->applyAuthorStyles();
 
     for (ScopedStyleResolver* scopedResolver = scopedResolverFor(element); scopedResolver; scopedResolver = scopedResolver->parent()) {
-        if (scopedResolver->treeScope() == treeScope || (applyAuthorStyles && scopedResolver->treeScope() == document))
+        if (scopedResolver->treeScope() == treeScope || (applyAuthorStyles && scopedResolver->treeScope() == &document))
             resolvers.append(scopedResolver);
     }
 }