Add Microsoft to OWNERS list for OpenXR and WMR related files

Remove ddorwin from the OWNERS file since he no longer works on
VR.

Change-Id: Idb8fdcc8e55295d2be23c1f172e88b2a1ac75d2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014145
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734096}

device/vr/openxr/OWNERS

+alcooper@chromium.org
+bajones@chromium.org
+rafael.cintron@microsoft.com
\ No newline at end of file

device/vr/windows_mixed_reality/OWNERS

+alcooper@chromium.org
+bajones@chromium.org
+rafael.cintron@microsoft.com
\ No newline at end of file

third_party/openxr/OWNERS

 alcooper@chromium.org
-ddorwin@chromium.org
+rafael.cintron@microsoft.com
 
 # TEAM: xr-dev@chromium.org
 # COMPONENT: Internals>XR