Change all vr/latest to xr

Renaming WebVR 2 code to WebXR as per community group desision.

Bug: 670510
Change-Id: Ia85cfe584afa3e21d4634414748e11f3c9b9bfc3
Reviewed-on: https://chromium-review.googlesource.com/823424


Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524408}