Currently, Chrome spins up most XR runtimes when an XR-using page loads. In addition to wasting resources, this could also cause runtime-specific UI to be displayed.

Instead, runtimes should only be loaded when they are about to be used. Specifically, that means when a WebXR session that will use that runtime is created.

According to billorr, we no longer do this for GVR and need to switch other devices to this pattern.

This issue tracks the desired end state across all platforms. We may want to have other issues blocking it.

A related issue is that we should not use HMD runtimes for inline VR (issue 891552).