Initialize XrInstance variable

XrInstance not being initialized to XR_NULL_HANDLE caused it to have
undefined data which resulted in a CHECK being triggered in
this).

OpenXrPlatformHelper: :DestroyInstance on Android (Windows overrides
Change-Id: Ib679e8a5c212ac1faa60c19f04de43498675c1dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4569028


Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149440}