Invoke WorkerScriptLoader's m_finishedCallback callback safely.

Source/core/workers/WorkerScriptLoader.h

Index: Source/core/workers/WorkerScriptLoader.h
diff --git a/Source/core/workers/WorkerScriptLoader.h b/Source/core/workers/WorkerScriptLoader.h
index c8c8f13ff34d6f1625eecf4bc867104cfc7cc9c4..62f679e8b03c495133855dcfe525fb0134eaac9d 100644
--- a/Source/core/workers/WorkerScriptLoader.h
+++ b/Source/core/workers/WorkerScriptLoader.h
@@ -57,6 +57,7 @@ public:
     // TODO: finishedCallback is not currently guaranteed to be invoked if used
     // from worker context and the worker shuts down in the middle of an
     // operation. This will cause leaks when we support nested workers.
+    // Note that callbacks could be invoked before loadAsynchronously() returns.
     void loadAsynchronously(ExecutionContext&, const KURL&, CrossOriginRequestPolicy, PassOwnPtr<Closure> responseCallback, PassOwnPtr<Closure> finishedCallback);
 
     void notifyError();
@@ -107,7 +108,6 @@ private:
     bool m_failed;
     unsigned long m_identifier;
     long long m_appCacheID;
-    bool m_finishing;
     OwnPtr<Vector<char>> m_cachedMetadata;
     WebURLRequest::RequestContext m_requestContext;
     RefPtr<ContentSecurityPolicy> m_contentSecurityPolicy;