The XRRigidTransform interface is a recent addition to the spec that is used in many places where we previously exposed a Float32Array matrix.

https://immersive-web.github.io/webxr/#xrrigidtransform-interface

The interface needs to be implemented in Chromium and used any place the spec calls for it:

 - Replace 'XRViewerPose.poseModelMatrix' with 'XRViewerPose.transform'
 - Replace 'XRInputPose.gripMatrix' with 'XRInputPose.gripTransform'
 - Add 'transform' to 'XRView'
 - Add 'transform' to 'XRReferenceSpaceEvent'
 - Add a constructor to 'XRRay' that takes in a 'XRRigidTransform'