Fix input-selection.html

Set the origin offset reference space on initialization. Otherwise, that
space will be null on the first call to onXRFrame and getViewerPose will
throw an exception, preventing the page from working.

Change-Id: Icf71ac3c8136e57906d01c3f8a4fdba8650f3495
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629693
Commit-Queue: Jacob DeWitt <jacde@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663922}

third_party/webxr_test_pages/webxr-samples/input-selection.html

@@ -432,6 +432,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
           // Save the session-specific reference space, and apply the
           // current player orientation/position as originOffset.
           setRefSpace(session, refSpace, false);
+          updateOriginOffset(session);
           session.requestReferenceSpace('viewer').then(function(viewerSpace){
             xrViewerSpace = viewerSpace;
             session.requestAnimationFrame(onXRFrame);