Bug 1546331 - Missing call to WaitForIsDebuggerRegistered r=asuth

Differential Revision: https://phabricator.services.mozilla.com/D52139

--HG--
extra : moz-landing-system : lando

dom/workers/WorkerPrivate.cpp

@@ -1468,6 +1468,12 @@ void WorkerPrivate::EnableDebugger() {
 void WorkerPrivate::DisableDebugger() {
   AssertIsOnParentThread();
 
+  // RegisterDebuggerMainThreadRunnable might be dispatched but not executed.
+  // Wait for its execution before unregistraion.
+  if (!NS_IsMainThread()) {
+    WaitForIsDebuggerRegistered(true);
+  }
+
   if (NS_FAILED(UnregisterWorkerDebugger(this))) {
     NS_WARNING("Failed to unregister worker debugger!");
   }