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Updated the OpenXR spec which changes the way extension methods are
queried. I separated the extension support enumeration and method
querying.

Updating the spec retains backwards and forward compatibility with
apps as that is mediated through the loader which is unchanged.
Extension lookup is now dynamic, and after this change, we can
implement AR features on OpenXR (e.g. hand tracking, anchors etc.)

Change-Id: I8a4ae718ca244c91b85e02f9441525a9b1416b58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406701Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: Lachlan Ford <laford@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#816823}