Bug 1842798 - Part 2: Remove use of MozPromise to wait for module imports to load r=smaug

This replaces the use of a promise to wait for all imports of a module to load
with a counter of reminaing imports in the parent and a pointer to the parent
that is waiting in the child. The parent request is updated immediately rather
than by dispatching a runnable.

Differential Revision: https://phabricator.services.mozilla.com/D183273

dom/workers/ScriptLoader.cpp

@@ -1210,6 +1210,13 @@ bool WorkerScriptLoader::EvaluateScript(JSContext* aCx,
 }
 
 void WorkerScriptLoader::TryShutdown() {
+  {
+    MutexAutoLock lock(CleanUpLock());
+    if (CleanedUp()) {
+      return;
+    }
+  }
+
   if (AllScriptsExecuted() && AllModuleRequestsLoaded()) {
     ShutdownScriptLoader(!mExecutionAborted, mMutedErrorFlag);
   }