Auto merge of #23485 - Manishearth:xrtest, r=asajeffrey

Basic XR Testing support

This adds support for the XRTest and FakeXRDeviceController APIs from https://github.com/immersive-web/webxr-test-api, and plugs them into the `mock` backend of rust-webvr.

Tested with [a modified webxr test page](https://github.com/immersive-web/webxr-test-api)

r? @jdm @asajeffrey

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23485)
<!-- Reviewable:end -->

components/script/dom/webidls/FakeXRDeviceController.webidl

@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://github.com/immersive-web/webxr-test-api/
+
+[Exposed=Window, Pref="dom.webxr.test"]
+interface FakeXRDeviceController {
+  // Creates and attaches a XRFrameOfReference of the type specified to the device.
+  // void setFrameOfReference(XRFrameOfReferenceType,  FakeXRFrameOfReferenceInit);
+
+  // // Sets the values to be used for subsequent
+  // // requestAnimationFrame() callbacks.
+  [Throws] void setViews(sequence<FakeXRViewInit> views);
+
+  [Throws] void setViewerOrigin(FakeXRRigidTransform origin);
+
+  // Simulates the user activating the reset pose on a device.
+  // void simulateResetPose();
+
+  // Simulates the platform ending the sessions.
+  // void simulateForcedEndSessions();
+
+  // Simulates devices focusing and blurring sessions.
+  // void simulateBlurSession(XRSession);
+  // void simulateFocusSession(XRSession);
+
+  // void setBoundsGeometry(Array<FakeXRBoundsPoint> boundsCoodinates)l
+
+  // Promise<FakeXRInputSourceController>
+  //     simulateInputSourceConnection(FakeXRInputSourceInit);
+};
+
+dictionary FakeXRViewInit {
+  required XREye eye;
+  // https://immersive-web.github.io/webxr/#view-projection-matrix
+  required sequence<float> projectionMatrix;
+  // https://immersive-web.github.io/webxr/#view-offset
+  required FakeXRRigidTransform viewOffset;
+};
+
+dictionary FakeXRBoundsPoint {
+  double x; double z;
+};
+
+dictionary FakeXRRigidTransform {
+    required sequence<float> position;
+    required sequence<float> orientation;
+};

components/script/dom/webidls/XR.webidl

@@ -28,3 +28,8 @@ dictionary XRSessionCreationOptions {
   XRSessionMode mode = "inline";
   // XRPresentationContext outputContext;
 };
+
+partial interface XR {
+  // https://github.com/immersive-web/webxr-test-api/
+  [SameObject, Pref="dom.webxr.test"] readonly attribute XRTest test;
+};

components/script/dom/webidls/XRTest.webidl

@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://github.com/immersive-web/webxr-test-api/
+
+[Exposed=Window, Pref="dom.webxr.test"]
+interface XRTest {
+  // Simulates connecting a device to the system.
+  // Used to instantiate a fake device for use in tests.
+  Promise<FakeXRDeviceController> simulateDeviceConnection(FakeXRDeviceInit init);
+
+  // // Simulates a device being disconnected from the system.
+  // Promise<void> simulateDeviceDisconnection(XRDevice);
+
+  // // Simulates a user activation (aka user gesture) for the current scope.
+  // // The activation is only guaranteed to be valid in the provided function and only applies to WebXR
+  // // Device API methods.
+  // void simulateUserActivation(Function);
+};
+
+dictionary FakeXRDeviceInit {
+    // TODO: Subject to change to match spec changes.
+    required boolean supportsImmersive;
+};