Auto merge of #12862 - servo:layout-new, r=emilio

added dom obj counting to decide sequential/parallel layout (#10110)

This is a rebased version of #11713

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #10110 (github issue number if applicable).
- [X] There are no tests for these changes because it's an optimization with no visible behavioral changes

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components/script/dom/window.rs

@@ -1084,6 +1084,7 @@ impl Window {
             window_size: window_size,
             script_join_chan: join_chan,
             query_type: query_type,
+            dom_count: self.Document().dom_count(),
         };
 
         self.layout_chan.send(Msg::Reflow(reflow)).unwrap();