Add support for Google Web Snippet bot to Bot-Aware ISR Fallback

[david-buck]

Describe the feature you'd like to request

Currently requests from the Google Web Snippet user agent:

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36 Google-PageRenderer Google (+https://developers.google.com/+/web/snippet/)

aren't served using fallback: blocking as other bots are. This can break previews for Android Messaging and Google Chat.

Describe the solution you'd like

Add the Google Web Snippet bot to the list of bots that triggers the blocking fallback.

Describe alternatives you've considered

1. Delaying render of the page
2. Serving generic page metadata

[styfle]

Thanks, we'll get this fixed #31521