https://codereview.chromium.org/2705293014/ introduces an enum type to describe which type of user agent is used for a particular NavigationItem in the iOS web layer (MOBILE, DESKTOP, or NONE).  The primary motivation for this is to have more accurate information in the model objects, as the user agent is not actually sent for native pages since they are not loaded via HTTP requests.  Additionally, this helps clean up logic for when to display the "Request Desktop/Mobile Site" option, and whether or not that option should be displayed.