chromeos: Makes mushrome use simplified display management

ash/shell_init_params.h

Index: ash/shell_init_params.h
diff --git a/ash/shell_init_params.h b/ash/shell_init_params.h
index 7a10cd73fca1aa504e356c05e46b919518fff6d7..2b4ab73acb441ee218c8078db24092ce58463e34 100644
--- a/ash/shell_init_params.h
+++ b/ash/shell_init_params.h
@@ -29,6 +29,8 @@ struct ASH_EXPORT ShellInitParams {
   ShellPort* shell_port = nullptr;
   // Shell takes ownership of |primary_window_tree_host|. This is only used
   // by mash.
+  // TODO(sky): once simplified display management is enabled for mash this
+  // should no longer be necessary. http://crbug.com/708287.
   aura::WindowTreeHostMus* primary_window_tree_host = nullptr;
   ShellDelegate* delegate = nullptr;
   ui::ContextFactory* context_factory = nullptr;