Use element id's for animations

cc/proto/element_id.proto

Index: cc/proto/element_id.proto
diff --git a/cc/proto/rect.proto b/cc/proto/element_id.proto
similarity index 54%
copy from cc/proto/rect.proto
copy to cc/proto/element_id.proto
index a0194b185d8954112eef235f63a9c52ea1050b5b..9e5cedd80a545e57009717cc25ebf0acc1bcb2c2 100644
--- a/cc/proto/rect.proto
+++ b/cc/proto/element_id.proto
@@ -4,14 +4,15 @@
 
 syntax = "proto2";
 
-import "point.proto";
-import "size.proto";
-
 option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
 
-message Rect {
-  optional cc.proto.Point origin = 1;
-  optional cc.proto.Size size = 2;
+// The message declared in this file corresponds to the class declared in
+// cc/animation/element_id.h
+
+// NEXT ID: 3
+message ElementId {
+  required int64 primary_id = 1;
+  required int64 secondary_id = 2;
 }