Update CSSAnimationUpdate and CSSAnimations to handle multiple animations with a given name

Source/core/animation/InertAnimation.h

Index: Source/core/animation/InertAnimation.h
diff --git a/Source/core/animation/InertAnimation.h b/Source/core/animation/InertAnimation.h
index e6cd2ea7c371e1bc46846c85232c0d5dd7a520d0..6cc2377d8239e6c37d7f4b856ba79f8d08184f8d 100644
--- a/Source/core/animation/InertAnimation.h
+++ b/Source/core/animation/InertAnimation.h
@@ -42,7 +42,7 @@ class InertAnimation FINAL : public TimedItem {
 public:
     static PassRefPtr<InertAnimation> create(PassRefPtr<AnimationEffect>, const Timing&);
     PassOwnPtr<AnimationEffect::CompositableValueMap> sample();
-    AnimationEffect* effect() { return m_effect.get(); }
+    AnimationEffect* effect() const { return m_effect.get(); }
 
 protected:
     virtual void updateChildrenAndEffects(bool) const OVERRIDE { };