Tidy up CompositorWorkerManager unit tests.

Source/modules/serviceworkers/ServiceWorkerGlobalScopeClient.cpp

Index: Source/modules/serviceworkers/ServiceWorkerGlobalScopeClient.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerGlobalScopeClient.cpp b/Source/modules/serviceworkers/ServiceWorkerGlobalScopeClient.cpp
index f42f49f4232a23a15a6111713127fb6fe0680579..415b92719567976dc6c01e8e233de0c40d86adf0 100644
--- a/Source/modules/serviceworkers/ServiceWorkerGlobalScopeClient.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerGlobalScopeClient.cpp
@@ -43,7 +43,9 @@ const char* ServiceWorkerGlobalScopeClient::supplementName()
 
 ServiceWorkerGlobalScopeClient* ServiceWorkerGlobalScopeClient::from(ExecutionContext* context)
 {
-    return static_cast<ServiceWorkerGlobalScopeClient*>(WillBeHeapSupplement<WorkerClients>::from(toWorkerGlobalScope(context)->clients(), supplementName()));
+    WorkerClients* clients = toWorkerGlobalScope(context)->clients();
+    ASSERT(clients);
+    return static_cast<ServiceWorkerGlobalScopeClient*>(WillBeHeapSupplement<WorkerClients>::from(clients, supplementName()));
 }
 
 void provideServiceWorkerGlobalScopeClientToWorker(WorkerClients* clients, PassOwnPtrWillBeRawPtr<ServiceWorkerGlobalScopeClient> client)