navigator.getVRDisplays return false on ios #123

Description:

Please describe your issue or feature request in detail.
Code and screenshots will help us diagnose the issue.

Additional Information:

• three.ar.js version:v0.1.8
• three.js version:r90
• Browser:safari

• WebARonARKit (iOS)
• WebARonARCore (Android)
• WebARonTango (Android)
• All
• Other (please specify)
  Device:iPhoneX ios12.4

it failed at here:
if (!navigator.getVRDisplays) {
resolve(null);
alert("!navigator.getVRDisplays");
return;
}