Google (and others) require that oAuth login flows go through an external browser (to avoid phishing scams etc.)
"In the coming months, we will no longer allow OAuth requests to Google in embedded browsers known as “web-views”, such as the WebView UI element on Android and UIWebView/WKWebView on iOS, and equivalents on Windows and OS X."
- https://developers.googleblog.com/2016/08/modernizing-oauth-interactions-in-native-apps.html
However, Apple is now rejecting binary updates...
SFSafariViewController and oAuth flows
"In the coming months, we will no longer allow OAuth requests to Google in embedded browsers known as “web-views”, such as the WebView UI element on Android and UIWebView/WKWebView on iOS, and equivalents on Windows and OS X."
- https://developers.googleblog.com/2016/08/modernizing-oauth-interactions-in-native-apps.html
However, Apple is now rejecting binary updates...
SFSafariViewController and oAuth flows