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

Source/core/animation/ElementAnimation.cpp

Index: Source/core/animation/ElementAnimation.cpp
diff --git a/Source/core/animation/ElementAnimation.cpp b/Source/core/animation/ElementAnimation.cpp
index ebf4be354bbcc50afee2d5aa9d2ac5f0e9a27a4d..defcbb10a0e5a2d9ff542d0de7a74f295f23c136 100644
--- a/Source/core/animation/ElementAnimation.cpp
+++ b/Source/core/animation/ElementAnimation.cpp
@@ -42,9 +42,8 @@ Animation* ElementAnimation::animate(Element& element, Vector<Dictionary> keyfra
     ASSERT(RuntimeEnabledFeatures::webAnimationsAPIEnabled());
 
     RefPtr<Animation> animation = Animation::create(&element, keyframeDictionaryVector, timingInput);
-    DocumentTimeline* timeline = element.document().timeline();
-    ASSERT(timeline);
-    timeline->play(animation.get());
+    DocumentTimeline& timeline = element.document().timeline();
+    timeline.play(animation.get());
 
     return animation.get();
 }
@@ -54,9 +53,8 @@ Animation* ElementAnimation::animate(Element& element, Vector<Dictionary> keyfra
     ASSERT(RuntimeEnabledFeatures::webAnimationsAPIEnabled());
 
     RefPtr<Animation> animation = Animation::create(&element, keyframeDictionaryVector, timingInput);
-    DocumentTimeline* timeline = element.document().timeline();
-    ASSERT(timeline);
-    timeline->play(animation.get());
+    DocumentTimeline& timeline = element.document().timeline();
+    timeline.play(animation.get());
 
     return animation.get();
 }
@@ -66,9 +64,8 @@ Animation* ElementAnimation::animate(Element& element, Vector<Dictionary> keyfra
     ASSERT(RuntimeEnabledFeatures::webAnimationsAPIEnabled());
 
     RefPtr<Animation> animation = Animation::create(&element, keyframeDictionaryVector);
-    DocumentTimeline* timeline = element.document().timeline();
-    ASSERT(timeline);
-    timeline->play(animation.get());
+    DocumentTimeline& timeline = element.document().timeline();
+    timeline.play(animation.get());
 
     return animation.get();
 }