Have Document::timeline() / Document::transitionTimeline() return a reference

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index c895725783d710aa74c21a8c291e882ea0cd6c36..961d0c07c9ca3cd72a3c32919362e42eb4149616 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -1001,8 +1001,8 @@ public:
     Locale& getCachedLocale(const AtomicString& locale = nullAtom);
 
     AnimationClock& animationClock() { return *m_animationClock; }
-    DocumentTimeline* timeline() const { return m_timeline.get(); }
-    DocumentTimeline* transitionTimeline() const { return m_transitionTimeline.get(); }
+    DocumentTimeline& timeline() const { return *m_timeline; }
+    DocumentTimeline& transitionTimeline() const { return *m_transitionTimeline; }
     CSSPendingAnimations& cssPendingAnimations() { return m_cssPendingAnimations; }
 
     void addToTopLayer(Element*, const Element* before = 0);