Don't leak a WebServiceWorker when 'current' races with context destruction.

Source/modules/serviceworkers/ServiceWorkerContainer.cpp

Index: Source/modules/serviceworkers/ServiceWorkerContainer.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerContainer.cpp b/Source/modules/serviceworkers/ServiceWorkerContainer.cpp
index e4f9c07f9ee146e53d139e3cdd0f4736ebff8535..8f64cd454ce526e251be8f3e408e567ba0e27fc9 100644
--- a/Source/modules/serviceworkers/ServiceWorkerContainer.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerContainer.cpp
@@ -142,8 +142,10 @@ ScriptPromise ServiceWorkerContainer::unregisterServiceWorker(ExecutionContext*
 
 void ServiceWorkerContainer::setCurrentServiceWorker(blink::WebServiceWorker* serviceWorker)
 {
-    if (!executionContext())
+    if (!executionContext()) {
+        delete serviceWorker;
         return;
+    }
     m_current = ServiceWorker::create(executionContext(), adoptPtr(serviceWorker));
 }