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As a result of TPAC there were some spec changes to the xr feature
policy (FP). These were made in:
https://github.com/immersive-web/webxr/pull/842
The following changes were needed to become spec compliant:

* rename FP from "xr" to "xr-spatial-tracking"
* supportsSession always resolves for "inline"
* supportsSession rejects for immersive-vr with a SecurityError if FP
  not set
* default features per mode are now required features for that mode
* requestSession does not unconditionally reject if FP is not set,
  instead resolving specific features now depends on FP status.
* While resolving features, if any required features depend on a missing
  FP, the requestSession call will fail with a "NotSupportedError"
* While resolving features, if any optional features depend on a missing
  FP, those features will be silently ignored.
* devicechange events will not fire for documents that do not have
  FP set.

Bug: 1003842
Change-Id: Ie9ae16b5c1d863b730e556b511c024bae8a4503c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808800
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Jacob DeWitt <jacde@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701740}