Auto merge of #20329 - gterzian:before_unload, r=cbrewster

Implement beforeunload event and infrastructure

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Implementation of:
1. https://html.spec.whatwg.org/multipage/#prompt-to-unload-a-document, and
2. https://html.spec.whatwg.org/multipage/#unload-a-document
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #10787 and fix #20485 and fix #20588 and fix #20496 (github issue number if applicable).

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- [ ] These changes do not require tests because _____

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components/script_traits/script_msg.rs

@@ -145,6 +145,8 @@ pub enum ScriptMsg {
     TouchEventProcessed(EventResult),
     /// A log entry, with the top-level browsing context id and thread name
     LogEntry(Option<String>, LogEntry),
+    /// Discard the document.
+    DiscardDocument,
     /// Notifies the constellation that this pipeline has exited.
     PipelineExited,
     /// Send messages from postMessage calls from serviceworker