Make all LayerAnimationElement::Create*Element return unique_ptr

ui/compositor/layer_animation_sequence.h

Index: ui/compositor/layer_animation_sequence.h
diff --git a/ui/compositor/layer_animation_sequence.h b/ui/compositor/layer_animation_sequence.h
index e050439da7eed4a7687f224b309596bb1448b05d..933ced79098ffe0702d7dd9c0b957d3ef994dfc2 100644
--- a/ui/compositor/layer_animation_sequence.h
+++ b/ui/compositor/layer_animation_sequence.h
@@ -11,7 +11,6 @@
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
-#include "base/memory/linked_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/time/time.h"
@@ -137,7 +136,7 @@ class COMPOSITOR_EXPORT LayerAnimationSequence
  private:
   friend class LayerAnimatorTestController;
 
-  typedef std::vector<linked_ptr<LayerAnimationElement> > Elements;
+  using Elements = std::vector<std::unique_ptr<LayerAnimationElement>>;
 
   FRIEND_TEST_ALL_PREFIXES(LayerAnimatorTest,
                            ObserverReleasedBeforeAnimationSequenceEnds);