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/script_thread.rs

@@ -1450,7 +1450,7 @@ impl ScriptThread {
             let scope_things = ServiceWorkerRegistration::create_scope_things(global_ref, script_url);
             let _ = self.constellation_chan.send(ConstellationMsg::RegisterServiceWorker(scope_things, scope));
         } else {
-            warn!("Registration failed for {}", pipeline_id);
+            warn!("Registration failed for {}", scope);
         }
     }