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/workerglobalscope.rs

@@ -397,12 +397,14 @@ impl WorkerGlobalScope {
     }
 
     pub fn script_chan(&self) -> Box<ScriptChan + Send> {
-        let dedicated =
-            self.downcast::<DedicatedWorkerGlobalScope>();
+        let dedicated = self.downcast::<DedicatedWorkerGlobalScope>();
+        let service_worker = self.downcast::<ServiceWorkerGlobalScope>();
         if let Some(dedicated) = dedicated {
             return dedicated.script_chan();
+        } else if let Some(service_worker) = service_worker {
+            return service_worker.script_chan();
         } else {
-            panic!("need to implement a sender for SharedWorker/ServiceWorker")
+            panic!("need to implement a sender for SharedWorker")
         }
     }