ServiceWorker: Fix data race in shutdown sequence

Source/web/WebEmbeddedWorkerImpl.cpp

Index: Source/web/WebEmbeddedWorkerImpl.cpp
diff --git a/Source/web/WebEmbeddedWorkerImpl.cpp b/Source/web/WebEmbeddedWorkerImpl.cpp
index f9cdbfde727cf76e5b46101e44ee129882f6fdd9..8dc72fbc5e0e1a35535053f97157ce227a1cfde7 100644
--- a/Source/web/WebEmbeddedWorkerImpl.cpp
+++ b/Source/web/WebEmbeddedWorkerImpl.cpp
@@ -51,6 +51,7 @@
 #include "platform/network/ContentSecurityPolicyParsers.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebURLRequest.h"
+#include "public/platform/WebWaitableEvent.h"
 #include "public/web/WebDevToolsAgent.h"
 #include "public/web/WebServiceWorkerContextClient.h"
 #include "public/web/WebServiceWorkerNetworkProvider.h"
@@ -171,6 +172,11 @@ WebEmbeddedWorkerImpl::WebEmbeddedWorkerImpl(
 
 WebEmbeddedWorkerImpl::~WebEmbeddedWorkerImpl()
 {
+    if (m_workerThread) {
+        ASSERT(m_workerThread->terminated());
+        m_workerThread->terminationEvent()->wait();
+    }
+
     ASSERT(runningWorkerInstances().contains(this));
     runningWorkerInstances().remove(this);
     ASSERT(m_webView);