Loosen some OpenXR validations

Running the OpenXR mock failed with the sandbox disabled because the
number of requested extensions did not match the number of supported
extensions. This isn't a strict requirement of the runtime, and really,
we should only care that all enabled extensions are actually supported,
which is validated by the next check.

Change-Id: I96b3961c886ff16642c3c3f2005e28e5a60b7b30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391216


Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804418}