[Oilpan] Apply RefCountedGarbageCollectedEventTarget on AbstractWorker

Source/core/workers/WorkerMessagingProxy.cpp

Index: Source/core/workers/WorkerMessagingProxy.cpp
diff --git a/Source/core/workers/WorkerMessagingProxy.cpp b/Source/core/workers/WorkerMessagingProxy.cpp
index 228b63f2c06ab3701369fcccec7e8befc0ea8e05..e3619d6033acebe5781a0abc24435e0242e03dd5 100644
--- a/Source/core/workers/WorkerMessagingProxy.cpp
+++ b/Source/core/workers/WorkerMessagingProxy.cpp
@@ -202,7 +202,11 @@ void WorkerMessagingProxy::workerThreadCreated(PassRefPtr<WorkerThread> workerTh
 
 void WorkerMessagingProxy::workerObjectDestroyed()
 {
-    m_workerObject = nullptr;
+    // workerObjectDestroyed() is called in InProcessWorkerBase's destructor.
+    // Thus it should be guaranteed that a weak pointer m_workerObject has been cleared
+    // before this method gets called.
+    ASSERT(!m_workerObject);
+
     m_executionContext->postTask(FROM_HERE, createCrossThreadTask(&WorkerMessagingProxy::workerObjectDestroyedInternal, this));
 }