[OpenXR] Add build flag and feature flag.

Part 1/5 from splitting crrev.com/c/1629016.

Bug: 974396
Change-Id: Ic882049650b274cdd8d83315d39e5d80b9ebcb79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1661272
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Patrick To <patrto@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#669533}
8 files changed