Make XR/VR presenting code async
Previously we only pretended to be async, we returned promises that were already resolved because we synchronously blocked on the channel.
This:
- Moves XR presentation to session creation (where it belongs, see https://github.com/immersive-web/webxr/issues/453)
- Factors out common presentation code in a way that supports asynchronously resolving promises
- Uses this for `VRDisplay::RequestPresent()` and `XR::RequestSession()`
r? @jdm
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