Auto merge of #22528 - Manishearth:webxr, r=jdm,MortimerGoro

Preliminary WebXR support

This implements just enough WebXR to display to 3DOF devices in
immersive mode only.

Couple missing things:

 - [ ] Handling reference spaces (even if just supporting eye-level spaces)
 - [x] Spec links
 - [ ] We enter immersive mode when baseLayer is set, but it seems like
we're supposed to do this when requestSession is called (https://github.com/immersive-web/webxr/issues/453)
 - [ ] VR/XR should block less (https://github.com/servo/servo/issues/22505)
 - [x] More pref-gating
 - [x] `views` is a method instead of an attribute because we don't support FrozenArray

<s>Once I add spec links and pref gating</s> this can be landed as-is for further experimentation.

r? @jdm @MortimerGoro

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python/tidy/servo_tidy/tidy.py

@@ -95,6 +95,7 @@ WEBIDL_STANDARDS = [
     "//svgwg.org/svg2-draft",
     "//wicg.github.io",
     "//webaudio.github.io",
+    "//immersive-web.github.io/",
     # Not a URL
     "// This interface is entirely internal to Servo, and should not be" +
     " accessible to\n// web pages."