cc:: Add transform nodes at boundaries of 3d rendering contexts

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 dc479a9402d45ac5d460a9e104cbc5a390ea1f92..ace8650e00b5c45b80210fd951c1b62182c99d66 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -92,6 +92,11 @@ struct CC_EXPORT TransformNodeData {
   // tree.
   int source_node_id;
 
+  // This id determines which 3d rendering context the node is in. 0 is a
+  // special value and indicates that the node is not in any 3d rendering
+  // context.
+  int sorting_context_id;
+
   // TODO(vollick): will be moved when accelerated effects are implemented.
   bool needs_local_transform_update : 1;