Uses a more reliable pattern to match the iOS major version number, also...

... implements a fallback if none is found. See listing for details: http://www.webapps-online.com/online-tools/user-agent-strings/dv/operatingsystem51849/ios

The issue appeared when testing on the iOS 7 simulator and on a device where the UserAgent would of the form:
Mozilla/5.0 (iPad; CPU OS 7_0 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Mobile/11A465

The pattern matching the iOS major version would seek /Version\/(\d+)/, which wouldn't match. The new pattern matches /OS (\d+)/i, and defaults to using viewport-units-buggyfill if not matched.
Oops, sorry for the Chrome thing, didn't realize you were talking about the UserAgent until it was too late. :)