Summary:PR for https://github.com/facebook/react-native/issues/5958. The viewport meta tags if present, are overridden from the page and it is rendered according to the screen size. An example has been added in the Web View section of UIExplorer demo app.
Closes https://github.com/facebook/react-native/pull/6013
Differential Revision: D2953940
Pulled By: nicklockwood
fb-gh-sync-id: 012769f3a2a3f7dc942b60de02a9d1b80a27236e
shipit-source-id: 012769f3a2a3f7dc942b60de02a9d1b80a27236e