Add 'allow="vr"' Feature Policy attribute to iframe to permit WebVR i…

…n cross-origin iframes

immersive-web/webxr#86
https://developers.google.com/web/updates/2017/09/webvr-origin-trial-chrome-62
https://bugs.chromium.org/p/chromium/issues/detail?id=666767

src/api/player.js

@@ -163,6 +163,7 @@ Player.prototype.createIframe_ = function(contentInfo) {
   var iframe = document.createElement('iframe');
   iframe.setAttribute('allowfullscreen', true);
   iframe.setAttribute('scrolling', 'no');
+  iframe.setAttribute('allow', 'vr');
   iframe.style.border = 0;
 
   // Handle iframe size if width and height are specified.