Remove ActiveDOMObject::willStop, and introduce WorkerGlobalScope::TerminationObserver.

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 158e16e68bc86053119bfe3bd3d8d246102a4666..0c888e3ba0e1559611935d1a192e7b5c81c16efe 100644
--- a/Source/core/workers/WorkerThread.cpp
+++ b/Source/core/workers/WorkerThread.cpp
@@ -232,7 +232,7 @@ void WorkerThread::stop()
     // Ensure that tasks are being handled by thread event loop. If script execution weren't forbidden, a while(1) loop in JS could keep the thread alive forever.
     if (m_workerGlobalScope) {
         m_workerGlobalScope->script()->scheduleExecutionTermination();
-        m_workerGlobalScope->willStopActiveDOMObjects();
+        m_workerGlobalScope->wasRequestedToTerminate();
         m_runLoop.postTaskAndTerminate(WorkerThreadShutdownStartTask::create());
         return;
     }