Use element id's for animations

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index 4264a1c6962fb911ae170753e3f01bf2751dce09..2bbc9be9d0b780764075223759b5d10d5a6d80ea 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -11,6 +11,7 @@
 #include <unordered_map>
 #include <vector>
 
+#include "cc/animation/element_id.h"
 #include "cc/base/cc_export.h"
 #include "cc/base/synced_property.h"
 #include "ui/gfx/geometry/rect_f.h"
@@ -324,7 +325,7 @@ struct CC_EXPORT ScrollNodeData {
   bool should_flatten;
   bool user_scrollable_horizontal;
   bool user_scrollable_vertical;
-  int element_id;
+  ElementId element_id;
   int transform_id;
   // Number of drawn layers pointing to this node or any of its descendants.
   int num_drawn_descendants;