Launch Visual studio internal web browser

Hi ,

I have created a add in for the TFS workitems , so i would like to launch Active visual studio internal explorer when i click on button in the add in. I was looked in google i got different suggestion but non of them worked. I was working with below code but the "Service" is always getting null. 

                IVsWindowFrame vsFrame;
                var service = Package.GetGlobalService(typeof(SVsWebBrowsingService)) as IVsWebBrowsingService;
                if (service != null)
                {
                    service.Navigate("www.google.com", 0, out vsFrame);
                    return true;
                }

Thanks,

Ravindra.