Replace originOffset public member with getOffsetReferenceSpace method

Remove originOffset as a public member of XRReferenceSpace. Add
getOffsetReferenceSpace() method for creating a new XRReferenceSpace
instead of mutating originOffset.

Bug: 958440
Change-Id: I9af336112df7a359a6ac02bd8d30e554ae540511
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595131
Commit-Queue: Will Cassella <cassew@google.com>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659786}

webxr/xrFrame_getPose.https.html

@@ -37,7 +37,7 @@
       let space2 = spaces[1];
 
       // Rotate 90 degrees about x axis, then move 1 meter along y axis.
-      space1.originOffset = new XRRigidTransform(
+      space1 = space1.getOffsetReferenceSpace(new XRRigidTransform(
         DOMPointReadOnly.fromPoint({
           x : 0,
           y : 1,
@@ -50,10 +50,10 @@
           z : 0,
           w : Math.cos(radians / 2)
         })
-      );
+      ));
 
       // Rotate 90 degrees about z axis, then move 1 meter along x axis.
-      space2.originOffset = new XRRigidTransform(
+      space2 = space2.getOffsetReferenceSpace(new XRRigidTransform(
         DOMPointReadOnly.fromPoint({
           x : 1,
           y : 0,
@@ -66,7 +66,7 @@
           z : Math.sin(radians / 2),
           w : Math.cos(radians / 2)
         })
-      );
+      ));
 
       let space1_from_space2 = xrFrame.getPose(space1, space2);
       const EXPECTED_POSE_MATRIX = [