When there is a custom reality installed, WebVR will not present

[blairmacintyre]

In Firefox Nightly, it briefly presents and then stops presenting.

However there must be some sort of race condition, because one time (with a panorama) it presented and stayed in stereo. On the panoramas sample, switching to the High Museum shows the "Peaches" text, but it is incredibly jittery, because of the delay caused by sending the pose data through the reality iframe.

We cannot use custom realities in WebVR.

We should probably disable WebVR if (a) it's hardware webVR and (b) we are using a custom reality.