Auto merge of #22953 - asajeffrey:webvr-glwindow, r=paulrouget

Use a test VRDisplay that renders to a GL window

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Add a `dom.webvr.test` pref that registers a new VR display, which registers to a GL window.

The matching webvr PR is https://github.com/servo/rust-webvr/pull/66.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #22795
- [X] These changes do not require tests because we need a followup PR to support reftests which use the VR display

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components/servo/lib.rs

@@ -258,10 +258,11 @@ where
         // can't defer it after `create_constellation` has started.
         script::init();
 
+        let mut webvr_heartbeats = Vec::new();
         let webvr_services = if PREFS.is_webvr_enabled() {
             let mut services = VRServiceManager::new();
             services.register_defaults();
-            window.register_vr_services(&mut services);
+            window.register_vr_services(&mut services, &mut webvr_heartbeats);
             Some(services)
         } else {
             None
@@ -308,6 +309,7 @@ where
                 webrender,
                 webrender_document,
                 webrender_api,
+                webvr_heartbeats,
             },
         );