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

Source/core/dom/ActiveDOMObject.h

Index: Source/core/dom/ActiveDOMObject.h
diff --git a/Source/core/dom/ActiveDOMObject.h b/Source/core/dom/ActiveDOMObject.h
index d51cac9e2eb2606bd11712102c359a50d50e7350..6adfb60ee6ed3eaf84275364fec69c352c3067b2 100644
--- a/Source/core/dom/ActiveDOMObject.h
+++ b/Source/core/dom/ActiveDOMObject.h
@@ -55,9 +55,6 @@ public:
     // which don't need special treatment can skip implementation.
     virtual void suspend();
     virtual void resume();
-    // willStop is called when stop() for the owner worker is called. It's not
-    // called if the owner is a Document. It's ok to post a task to the context.
-    virtual void willStop();
     virtual void stop();
 
     void didMoveToNewExecutionContext(ExecutionContext*);