[cc] Change semantics of fixed-pos container layer

cc/trees/property_tree_builder.cc

Index: cc/trees/property_tree_builder.cc
diff --git a/cc/trees/property_tree_builder.cc b/cc/trees/property_tree_builder.cc
index 73197741d39ad4229075a9492c8e1bb62121eac8..663ee3b1676179cea62e64b535826543a0f3326f 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -396,13 +396,7 @@ bool AddTransformNodeIfNeeded(
         layer == data_from_ancestor.inner_viewport_scroll_layer;
     data_for_children->affected_by_outer_viewport_bounds_delta =
         layer == data_from_ancestor.outer_viewport_scroll_layer;
-    if (is_scrollable) {
-      DCHECK(!is_root);
-      DCHECK(Transform(layer).IsIdentity());
-      data_for_children->transform_fixed_parent = Parent(layer);
-    } else {
-      data_for_children->transform_fixed_parent = layer;
-    }
+    data_for_children->transform_fixed_parent = layer;
   }
   data_for_children->transform_tree_parent = layer;