enterXRAsync override in EngineHook.ts fails TypeScript type checking

[ryantrem]

EngineHook.ts has an override of WebXRExperienceHelper.enterXRAsync to force the clear color. To do this, it accesses the scene property of WebXRExperienceHelper, but this is private (I'm guessing it used to be public). This results in a TypeScript type checking error. We should:

1. Ideally just remove this if @bghgary's big refactor fixes it (I'm guessing it does).
2. If we must keep it, we should change this.scene to this.sessionManager.scene.

[chrisfromwork]

Testing this out locally, it seems like @bghgary 's changes didn't change things to the point that we can remove this auto clear logic. I'll put out a change updating the line for typescript type checking