CC Animation: Use ElementId to attach CC animation players.

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index 26560d1d5fbd1a1f287e921c9c1323649cb738c7..fb19ca58bcda247e01a67ffcda816f8edbd0bc5f 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -22,6 +22,7 @@
 #include "cc/layers/texture_layer_client.h"
 #include "cc/resources/texture_mailbox.h"
 #include "cc/surfaces/surface_id.h"
+#include "cc/trees/element_id.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/core/SkRegion.h"
 #include "ui/compositor/compositor.h"
@@ -88,6 +89,8 @@ class COMPOSITOR_EXPORT Layer
                      scoped_refptr<cc::Layer> root_layer);
   void ResetCompositor();
 
+  static cc::ElementId NextElementId();
+
   LayerDelegate* delegate() { return delegate_; }
   void set_delegate(LayerDelegate* delegate) { delegate_ = delegate; }