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/webidls/ServiceWorker.webidl

@@ -7,7 +7,7 @@
 interface ServiceWorker : EventTarget {
   readonly attribute USVString scriptURL;
   readonly attribute ServiceWorkerState state;
-  //[Throws] void postMessage(any message/*, optional sequence<Transferable> transfer*/);
+  [Throws] void postMessage(any message/*, optional sequence<Transferable> transfer*/);
 
   // event
   attribute EventHandler onstatechange;