Right now VrBrowserRendererThreadWin exists while a page is presenting, but VR-UI objects (VRUiBrowserInterface, Ui, BrowserUiInterface, etc.) only exist while UI is shown.

A few things need to be adjusted.
To support UI when a page isn't presenting (notifications that a page has stopped presentation, timeout spinner), lifetime shouldn't be restricted to when presenting.
As an optimization, we could be ready to show UI when presentation starts, potentially cutting down on time to parse shaders, and initialize the VR-UI.
Testing may be simpler if vr-ui objects exist even when UI isn't shown.