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

public/web/WebFrameClient.h

Index: public/web/WebFrameClient.h
diff --git a/public/web/WebFrameClient.h b/public/web/WebFrameClient.h
index 17e7770b70bf3fa18b29158ec3d24353233ff509..e44c22bf5fd9976a9e552019b607ea2a59533e5d 100644
--- a/public/web/WebFrameClient.h
+++ b/public/web/WebFrameClient.h
@@ -572,6 +572,10 @@ public:
     // ServiceWorker.
     virtual bool isControlledByServiceWorker(WebDataSource&) { return false; }
 
+    // Returns an identifier of the service worker controlling the document
+    // associated with the WebDataSource.
+    virtual int64_t serviceWorkerID(WebDataSource&) { return -1; }
+
 protected:
     virtual ~WebFrameClient() { }
 };