Auto merge of #14173 - asajeffrey:script-thread-stores-top-level-frame-id, r=paulrouget

Report panics using the top-level frame id rather than the pipeline id

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At the moment, we report panics from script and layout using the root pipeline id. Once we are sharing more script threads, there won't be a root pipeline id any more. The plan is only to share script threads in the same tab, so there will be a top-level frame id that all the pipelines have in common. This PR reports panics using that top-level frame id.

This mostly makes a difference to the browser API: rather than targeting a `mozbrowsererror` event at the root iframe of the script thread that panicked, it targets it at the containing mozbrowser iframe.

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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 do not require tests because we don't test crash reporting

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components/script/script_thread.rs

@@ -518,8 +518,8 @@ impl ScriptThreadFactory for ScriptThread {
         thread::spawn_named(format!("ScriptThread {:?}", state.id),
                             move || {
             thread_state::initialize(thread_state::SCRIPT);
-            PipelineId::install(state.id);
             PipelineNamespace::install(state.pipeline_namespace_id);
+            FrameId::install(state.top_level_frame_id);
             let roots = RootCollection::new();
             let _stack_roots_tls = StackRootTLS::new(&roots);
             let id = state.id;