Tidy up CompositorWorkerManager unit tests.

Source/core/workers/WorkerThreadStartupData.h

Index: Source/core/workers/WorkerThreadStartupData.h
diff --git a/Source/core/workers/WorkerThreadStartupData.h b/Source/core/workers/WorkerThreadStartupData.h
index 7b83d844a6475b3666a2f1f5d7bd39e0011959be..bb7f75075d344ac72b93ed759bbe9d53f324492e 100644
--- a/Source/core/workers/WorkerThreadStartupData.h
+++ b/Source/core/workers/WorkerThreadStartupData.h
@@ -84,7 +84,8 @@ public:
     //
     // Hence, CrossThreadPersistent<> is required to allow finalization
     // to happen on a thread different than the thread creating the
-    // persistent reference.
+    // persistent reference. If the worker thread creation context
+    // supplies no extra 'clients', m_workerClients can be left as empty/null.
     OwnPtrWillBeCrossThreadPersistent<WorkerClients> m_workerClients;
 
     V8CacheOptions m_v8CacheOptions;