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

Source/core/dom/shadow/ShadowRoot.cpp

Index: Source/core/dom/shadow/ShadowRoot.cpp
diff --git a/Source/core/dom/shadow/ShadowRoot.cpp b/Source/core/dom/shadow/ShadowRoot.cpp
index 6269a5cb0dd2da4b0dfdad93db66d2440ba1234a..4554cfa6cd71991c2c2ad1009f702be036fef951 100644
--- a/Source/core/dom/shadow/ShadowRoot.cpp
+++ b/Source/core/dom/shadow/ShadowRoot.cpp
@@ -162,7 +162,7 @@ void ShadowRoot::recalcStyle(StyleChange change)
     // ShadowRoot doesn't support custom callbacks.
     ASSERT(!hasCustomStyleCallbacks());
 
-    StyleResolver* styleResolver = document()->styleResolver();
+    StyleResolver* styleResolver = document().styleResolver();
     styleResolver->pushParentShadowRoot(this);
 
     if (!attached()) {
@@ -252,7 +252,7 @@ void ShadowRoot::setResetStyleInheritance(bool value)
 
 void ShadowRoot::attach(const AttachContext& context)
 {
-    StyleResolver* styleResolver = document()->styleResolver();
+    StyleResolver* styleResolver = document().styleResolver();
     styleResolver->pushParentShadowRoot(this);
     DocumentFragment::attach(context);
     styleResolver->popParentShadowRoot(this);