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_layout_interface/message.rs

@@ -126,6 +126,8 @@ pub struct ScriptReflow {
     pub script_join_chan: Sender<()>,
     /// The type of query if any to perform during this reflow.
     pub query_type: ReflowQueryType,
+    /// The number of objects in the dom #10110
+    pub dom_count: u32,
 }
 
 impl Drop for ScriptReflow {