Make Element::ensureActiveAnimations() return a reference

Source/core/animation/Animation.cpp

Index: Source/core/animation/Animation.cpp
diff --git a/Source/core/animation/Animation.cpp b/Source/core/animation/Animation.cpp
index f77e9f63c8fd9f64bb8adc920520b279b3f28b65..bc9b5ac92222bc7fe7368605deae6f855844d292 100644
--- a/Source/core/animation/Animation.cpp
+++ b/Source/core/animation/Animation.cpp
@@ -329,7 +329,7 @@ Animation::Animation(PassRefPtr<Element> target, PassRefPtrWillBeRawPtr<Animatio
 void Animation::didAttach()
 {
     if (m_target)
-        m_target->ensureActiveAnimations()->players().add(player());
+        m_target->ensureActiveAnimations().players().add(player());
 }
 
 void Animation::willDetach()
@@ -342,7 +342,7 @@ void Animation::willDetach()
 
 static AnimationStack& ensureAnimationStack(Element* element)
 {
-    return element->ensureActiveAnimations()->defaultStack();
+    return element->ensureActiveAnimations().defaultStack();
 }
 
 void Animation::applyEffects(bool previouslyInEffect)