Auto merge of #12910 - creativcoder:swsender, r=jdm

Implement postMessage for ServiceWorkers

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Fixes #12773
r? @jdm

Changes:
* Implements `postMessage` on `ServiceWorker` object.
* Removes unused channels from sw and their scopes.
* Fixes a crash when calling `scope.script_chan()` in sw-scopes event handling.

---
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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 #12773

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- [X] There are tests for these changes at `tests/html/service-worker`

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components/script/dom/serviceworkercontainer.rs

@@ -95,10 +95,9 @@ impl ServiceWorkerContainerMethods for ServiceWorkerContainer {
             return Err(Error::Type("Scope URL contains forbidden characters".to_owned()));
         }
 
-        let scope_str = scope.as_str().to_owned();
         let worker_registration = ServiceWorkerRegistration::new(self.global().r(),
                                                                  script_url,
-                                                                 scope_str.clone(),
+                                                                 scope.clone(),
                                                                  self);
         ScriptThread::set_registration(scope, &*worker_registration, self.global().r().pipeline_id());
         Ok(worker_registration)