Auto merge of #11427 - mbrubeck:under-display-none, r=pcwalton

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.

r? @pcwalton

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

@@ -80,6 +80,7 @@
                                    _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