Google, long time ago, made the good decision to make the browser THE user interface. They did not have their own operating system. So to be platform independent choosing for the browser was a smart choice. But than smart-phones came along. And where to put ads (the Google money maker) on a smartphone app? Well in the browser again. But in the begin days apps where depending on poor network capacity and not browser based.
Today that is changing back again. And Google decided to make mobile friendly webpages better ranked on searching from a mobile device. (http://googlewebmastercentral.blogspot.nl/2015/02/finding-more-mobile-friendly-search.html)
If Google does not want to lose ad business on portable devices, they rapidly introduce aPhone and aTablet for Android smart phone and Android Tablets as a reply on navigator.platform.
And for Chromecast it should reply 'aWall' (Android Wall)