If we could manage references from the TLR to display root surfaces then it would simplify things in the WS. FrameGenerator wouldn't need to handle adding and removing TLR references and DisplayCompositor wouldn't need to pass the TLR to DisplayCompositorClient.

This would also allow fixing removing the TLR reference to a display root before the compositor frame sink is destroyed. We remove references over the MojoCompositorFrameSink interface, so it's impossible to remove a reference after the interface connection is closed.