Implement OpenXR Rendering Loop

To try out this code with the Microsoft OpenXR Runtime in Chromium, perform the following steps.
    - Install 'Mixed Reality OpenXR Developer Preview' from the Microsoft Store
    - If prompted also download the compatibility pack

Add the following to custom_vars in .gclient, located at the root of the chromium clone:
    "checkout_openxr": True
Add the following to args.gn, located in the out folder:
    enable_openxr = true

gclient sync and build

Once you've built and installed Chromium go to about:flags and:
    - Enable WebVR or WebXR
    - Enable 'OpenXR Support'

Bug: 976430,976436
Change-Id: I8a7c69b3a13349aa9e083d6206121b69a1f724cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688443
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Patrick To <patrto@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#678528}
22 files changed