Stop generating flows under display: none.

Because this is a bottom-up traversal it can generates flows and throw them away. To prevent that, this cascades an internal `-servo-under-display-none` property and then checks that during flow construction.  Fixes #1536.

components/style/properties/longhand/box.mako.rs

@@ -78,6 +78,7 @@
                                    _error_reporter: &mut StdBox<ParseErrorReporter + Send>) {
             longhands::_servo_display_for_hypothetical_box::derive_from_display(context);
             longhands::_servo_text_decorations_in_effect::derive_from_display(context);
+            longhands::_servo_under_display_none::derive_from_display(context);
         }
     % endif