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

Source/core/animation/DocumentAnimations.cpp

Index: Source/core/animation/DocumentAnimations.cpp
diff --git a/Source/core/animation/DocumentAnimations.cpp b/Source/core/animation/DocumentAnimations.cpp
index 8ff672369032134c29455775c3bf0aa37d907a58..8bdff82bb1f089f15688d8fccac851b995005fd0 100644
--- a/Source/core/animation/DocumentAnimations.cpp
+++ b/Source/core/animation/DocumentAnimations.cpp
@@ -48,22 +48,22 @@ namespace {
 
 void updateAnimationTiming(Document& document)
 {
-    document.timeline()->serviceAnimations();
-    document.transitionTimeline()->serviceAnimations();
+    document.timeline().serviceAnimations();
+    document.transitionTimeline().serviceAnimations();
 }
 
 } // namespace
 
 void DocumentAnimations::dispatchAnimationEvents(Document& document)
 {
-    document.timeline()->dispatchEvents();
-    document.transitionTimeline()->dispatchEvents();
+    document.timeline().dispatchEvents();
+    document.transitionTimeline().dispatchEvents();
 }
 
 void DocumentAnimations::dispatchAnimationEventsAsync(Document& document)
 {
-    document.timeline()->dispatchEventsAsync();
-    document.transitionTimeline()->dispatchEventsAsync();
+    document.timeline().dispatchEventsAsync();
+    document.transitionTimeline().dispatchEventsAsync();
 }
 
 void DocumentAnimations::updateAnimationTimingForAnimationFrame(Document& document, double monotonicAnimationStartTime)
@@ -77,7 +77,7 @@ void DocumentAnimations::updateAnimationTimingForGetComputedStyle(Node& node, CS
     if (!node.isElementNode())
         return;
     const Element& element = toElement(node);
-    if (element.document().timeline()->hasOutdatedPlayer()) {
+    if (element.document().timeline().hasOutdatedPlayer()) {
         updateAnimationTiming(element.document());
         return;
     }