navigator.userAgent.toLowerCase() crashes for React Native

[escwald]

React Native depends on debug and React Native does not have navigator.userAgent, which makes function useColors() (in browser.js) crash.

More specific, it is these lines that crashes.
(navigator.userAgent.toLowerCase().match(/firefox\/(\d+)/) && parseInt(RegExp.$1, 10) >= 31) || (navigator.userAgent.toLowerCase().match(/applewebkit\/(\d+)/));

Maybe do add a check for userAgent? Like this?
(navigator.userAgent && navigator.userAgent.toLowerCase().match(/firefox\/(\d+)/) && parseInt(RegExp.$1, 10) >= 31) || (navigator.userAgent && navigator.userAgent.toLowerCase().match(/applewebkit\/(\d+)/));

cc @ralphtheninja