As of March 2019 Apple released update 12.2 which made a change in Safari that causes the automatic tracking of movements to not work in VR. To resolve this issue, there is a quick fix outlined below:

• Select "Settings"
• Scroll down and select "Safari"
• Toggle on "Motion and Orientation Access"

Apple has made this change as a privacy feature, to keep your device as secure as possible. You may want to disable this feature once you have completed your VR session.