Worker: Introduce an observation mechanism for WorkerThread termination

third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.cpp

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/workers/WorkerThreadLifecycleObserver.h"
+
+#include "core/workers/WorkerThread.h"
+#include "wtf/Assertions.h"
+#include "wtf/WTF.h"
+
+namespace blink {
+
+WorkerThreadLifecycleObserver::WorkerThreadLifecycleObserver(WorkerThreadContext* workerThreadContext)
+    : LifecycleObserver(workerThreadContext)
+    , m_wasContextDestroyedBeforeObserverCreation(workerThreadContext->m_wasContextDestroyed)
+{
+    DCHECK(isMainThread());
+}
+
+} // namespace blink