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

Source/core/page/animation/KeyframeAnimation.cpp

Index: Source/core/page/animation/KeyframeAnimation.cpp
diff --git a/Source/core/page/animation/KeyframeAnimation.cpp b/Source/core/page/animation/KeyframeAnimation.cpp
index da07f661aa071d343c5c68e8328f574db624df80..3777e3d831bae3a5677c650b7570d0ebf4d128e5 100644
--- a/Source/core/page/animation/KeyframeAnimation.cpp
+++ b/Source/core/page/animation/KeyframeAnimation.cpp
@@ -53,7 +53,7 @@ KeyframeAnimation::KeyframeAnimation(const CSSAnimationData* animation, RenderOb
 {
     // Get the keyframe RenderStyles
     if (m_object && m_object->node() && m_object->node()->isElementNode())
-        m_object->document()->styleResolver()->keyframeStylesForAnimation(toElement(m_object->node()), unanimatedStyle, m_keyframes);
+        m_object->document().styleResolver()->keyframeStylesForAnimation(toElement(m_object->node()), unanimatedStyle, m_keyframes);
 
     // Update the m_transformFunctionListValid flag based on whether the function lists in the keyframes match.
     validateTransformFunctionList();
@@ -287,7 +287,7 @@ void KeyframeAnimation::endAnimation()
 
 bool KeyframeAnimation::shouldSendEventForListener(Document::ListenerType listenerType) const
 {
-    return m_object->document()->hasListenerType(listenerType);
+    return m_object->document().hasListenerType(listenerType);
 }
 
 void KeyframeAnimation::onAnimationStart(double elapsedTime)