Fix HUD for WebForms with no viewstate

Not sure if you want this PR here, but this adds a page to the sample WebForms app that has no viewstate (to confirm this was breaking HUD), and then provides the necessary fix in `glimpse.js`. I'm not sure how you're managing `glimpse.js` these days, but I assume it's actually from Glimpse/Glimpse.Client, so I've made the fix there and opened a PR too: Glimpse/Glimpse.Client#19. Let me know if you want any changes.