WPT test to verify XRRigidTransform.matrix is SameObject

Bug: 995845
Change-Id: I830f5c9b1eac21b37f4ac534b0c862b8265265e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761437
Commit-Queue: Jacob DeWitt <jacde@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Auto-Submit: Jacob DeWitt <jacde@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688632}

third_party/blink/web_tests/external/wpt/webxr/xrRigidTransform_sameObject.https.html

@@ -22,11 +22,14 @@ let testFunction =
 
   let position = transform.position;
   let orientation = transform.orientation;
+  let matrix = transform.matrix;
   t.step(() => {
     assert_equals(position, transform.position,
       "XRRigidTransform.position returns the same object.");
     assert_equals(orientation, transform.orientation,
       "XRRigidTransform.orientation returns the same object.");
+    assert_equals(matrix, transform.matrix,
+      "XRRigidTransform.matrix returns the same object.");
   });
 
   resolve();