Add OpenXR Loader to third_party

The OpenXR API is a set of C APIs designed to build cross-platform XR
applications. In order to use OpenXR runtime, we need to use openxr_loader first.
"The loader is critical to detecting, exposing,
and possibly loading any available OpenXR runtime or API layer on the system".
(https://www.khronos.org/openxr)
Vulkan works in a similar manner to OpenXR where the application is responsible for
shipping code that does runtime detection.

The loader is built directly from source code.

Bug: 953821
Change-Id: Id88d73e420f49df1a69d82ae11e7a7922bb11b97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611652
Commit-Queue: Zheng Qin <zheqi@microsoft.com>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672368}