ui::WebDialogDelegate::OnDialogClosed() has a comment that says:

IMPORTANT: Implementations should delete |this| here (unless they've arranged for the delegate to be deleted in some other way, e.g. by registering it as a message handler in the WebUI object).

But it's not obvious if all delegate implementations are actually doing this. We should audit them and make it very clear what each delegate's lifetime / memory management model is.