[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..67105391f379ef23d58128de9fade091b037966c 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -393,16 +393,16 @@ bool AddTransformNodeIfNeeded(
 
   if (IsContainerForFixedPositionLayers(layer) || is_root) {
     data_for_children->affected_by_inner_viewport_bounds_delta =
-        layer == data_from_ancestor.inner_viewport_scroll_layer;
+        layer->id() == (data_from_ancestor.inner_viewport_scroll_layer
+                            ? data_from_ancestor.inner_viewport_scroll_layer
+                                  ->scroll_clip_layer_id()
+                            : Layer::INVALID_ID);
     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;
-    }
+        layer->id() == (data_from_ancestor.outer_viewport_scroll_layer
+                            ? data_from_ancestor.outer_viewport_scroll_layer
+                                  ->scroll_clip_layer_id()
+                            : Layer::INVALID_ID);
+    data_for_children->transform_fixed_parent = layer;
   }
   data_for_children->transform_tree_parent = layer;