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/constellation/constellation.rs

@@ -996,6 +996,13 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 debug!("constellation got store registration scope message");
                 self.handle_register_serviceworker(scope_things, scope);
             }
+            FromScriptMsg::ForwardDOMMessage(msg_vec, scope_url) => {
+                if let Some(ref mgr) = self.swmanager_chan {
+                    let _ = mgr.send(ServiceWorkerMsg::ForwardDOMMessage(msg_vec, scope_url));
+                } else {
+                    warn!("Unable to forward DOMMessage for postMessage call");
+                }
+            }
         }
     }