ServiceWorker: Make a way to retrieve a service worker version id (Blink)

Source/core/loader/EmptyClients.h

Index: Source/core/loader/EmptyClients.h
diff --git a/Source/core/loader/EmptyClients.h b/Source/core/loader/EmptyClients.h
index 7a6a3550afb198f570030f955cfd72ffa751e35f..b20e4ca62187a0843532dbf077aa0e6de7a4a242 100644
--- a/Source/core/loader/EmptyClients.h
+++ b/Source/core/loader/EmptyClients.h
@@ -248,6 +248,7 @@ public:
 
     virtual PassOwnPtr<blink::WebServiceWorkerProvider> createServiceWorkerProvider() override;
     virtual bool isControlledByServiceWorker(DocumentLoader&) override { return false; }
+    virtual int64_t serviceWorkerID(DocumentLoader&) override { return -1; }
     virtual PassOwnPtr<blink::WebApplicationCacheHost> createApplicationCacheHost(blink::WebApplicationCacheHostClient*) override;
 };