Introduce WorkerThread::willShutdown()

Source/core/workers/WorkerThread.cpp

Index: Source/core/workers/WorkerThread.cpp
diff --git a/Source/core/workers/WorkerThread.cpp b/Source/core/workers/WorkerThread.cpp
index e8c44a3c863da1942fe14243d6b8ba1f3218c482..be4ffb32c3f1661dfd14b6c8377ce348e32241b4 100644
--- a/Source/core/workers/WorkerThread.cpp
+++ b/Source/core/workers/WorkerThread.cpp
@@ -277,6 +277,7 @@ void WorkerThread::shutdown()
     // This should be called before we start the shutdown procedure.
     workerReportingProxy().willDestroyWorkerGlobalScope();
 
+    willShutdown();
     workerGlobalScope()->dispose();
 
     backingThread().removeTaskObserver(m_microtaskRunner.get());
@@ -456,6 +457,13 @@ v8::Isolate* WorkerThread::initializeIsolate()
     return isolate;
 }
 
+void WorkerThread::willShutdown()
+{
+    ASSERT(isCurrentThread());
+    ASSERT(m_isolate);
+    ASSERT(m_workerGlobalScope->script());
+}
+
 void WorkerThread::willDestroyIsolate()
 {
     ASSERT(isCurrentThread());