Root nodes for the duration of their CSS transitions.

This ensures that we can pass a node address as part of the asynchronous
transition end notification, making it safe to fire the corresponding
DOM event on the node from the script thread. Without explicitly rooting
this node when the transition starts, we risk the node being GCed before
the transition is complete.

components/script_layout_interface/rpc.rs

@@ -42,6 +42,8 @@ pub trait LayoutRPC {
     fn pending_images(&self) -> Vec<PendingImage>;
     /// Requests the list of nodes from the given point.
     fn nodes_from_point_response(&self) -> Vec<UntrustedNodeAddress>;
+    /// Requests the list of nodes that have just started CSS transitions in the last reflow.
+    fn newly_transitioning_nodes(&self) -> Vec<UntrustedNodeAddress>;
 
     fn text_index(&self) -> TextIndexResponse;
 }