Auto merge of #10862 - asajeffrey:only-the-chrome-survives, r=SimonSapin

Don't kill the root pipeline when randomly killing pipelines.

Useful for stress-testing the browser chrome, e.g. testing #10334.

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components/compositing/constellation.rs

@@ -1869,6 +1869,8 @@ impl<LTF: LayoutThreadFactory, STF: ScriptThreadFactory> Constellation<LTF, STF>
         if let Some((ref mut rng, _)) = self.random_pipeline_closure {
             if let Some(pipeline_id) = rng.choose(&*pipeline_ids) {
                 if let Some(pipeline) = self.pipelines.get(pipeline_id) {
+                    // Don't kill the root pipeline
+                    if pipeline.parent_info.is_none() { return; }
                     // Note that we deliberately do not do any of the tidying up
                     // associated with closing a pipeline. The constellation should cope!
                     info!("Randomly closing pipeline {}.", pipeline_id);