Safari View Controller is a new feature in iOS 9 that lets developers enable an in-app browser based on Safari that taps into many of the features that make using standalone Safari so great. Some of the benefits are the ability to use current logins, access the keychain and use autofill.
Previously, developers had to come up with their own browsers for their apps, which meant that users couldn’t tap into ongoing logins, the keychain, or use autofill when viewing the web through these browsers. That resulted in, as you can probably attest to, a less that ideal and disconnected user experience.
Apple has fixed this issue by allowing developers to use Safari View Controller in order to bridge the gap between the sites and data that they already use, and the apps that they love.
Unfortunately, not all apps yet support Safari View Controller. Some popular apps, like Gmail, Twitter, and WhatsApp, to name a few, still use their own built in browsers. SFSafariViewController all the things is a brand new tweak that aims to fix this issue.