Fix minor webxr-test.js bug

Bug: 863557
Change-Id: I84e913d238dedf278b070afd9f3c79674373fc44
Reviewed-on: https://chromium-review.googlesource.com/1150290Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577972}

third_party/WebKit/LayoutTests/external/wpt/resources/chromium/webxr-test.js

@@ -102,7 +102,7 @@ class MockDevice {
   // Test methods.
   setXRPresentationFrameData(poseMatrix, views) {
     if (poseMatrix == null) {
-      this.presentation_provider_.pose_ = null;
+      this.pose_ = null;
     } else {
       this.setPoseFromMatrix(poseMatrix);
     }