Update interfaces/webxr.idl (#17201)

Source: https://github.com/tidoust/reffy-reports/blob/fd61a58/whatwg/idl/webxr.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/542104807
diff --git a/interfaces/webxr.idl b/interfaces/webxr.idl
index 63327a3..9b48a0f 100644
--- a/interfaces/webxr.idl
+++ b/interfaces/webxr.idl
@@ -126,7 +126,7 @@
 interface XRRigidTransform {
   [SameObject] readonly attribute DOMPointReadOnly position;
   [SameObject] readonly attribute DOMPointReadOnly orientation;
-  [SameObject] readonly attribute Float32Array matrix;
+  readonly attribute Float32Array matrix;
   [SameObject] readonly attribute XRRigidTransform inverse;
 };
 
@@ -136,7 +136,7 @@
 interface XRRay {
   [SameObject] readonly attribute DOMPointReadOnly origin;
   [SameObject] readonly attribute DOMPointReadOnly direction;
-  [SameObject] readonly attribute Float32Array matrix;
+  readonly attribute Float32Array matrix;
 };
 
 [SecureContext, Exposed=Window] interface XRPose {