only run polyfill if nativeWebVR is not available

If you run the polyfill when native VR is available, you won't be able to put the player into VR mode properly. This should fix borismus/webvr-boilerplate#170 and videojs/videojs-vr#14

src/webvr-polyfill.js

@@ -36,7 +36,7 @@ function WebVRPolyfill() {
                                  navigator.getVRDisplays :
                                  null;
 
-  if (!this.nativeLegacyWebVRAvailable) {
+  if (!this.nativeLegacyWebVRAvailable && !this.nativeWebVRAvailable) {
     this.enablePolyfill();
     if (window.WebVRConfig.ENABLE_DEPRECATED_API) {
       this.enableDeprecatedPolyfill();