Add xr-spatial-tracking enum value + entry

[marcoscaceres]

closes #108

The following tasks have been completed:

• Modified Web platform tests

Implementation commitment:

• WebKit - implemented as a permission policy, but Permissions API is not yet implemented
• Blink - implemented
• Gecko - implemented, but experimental

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Preview | Diff

[marcoscaceres]

Gecko seems to implements this, but not through the Permissions API:
https://searchfox.org/mozilla-central/source/dom/vr/XRPermissionRequest.cpp

At least, they are not hooked up together.

@cabanier or @Manishearth, do you know if folks at Mozilla is still working on XR?

[cabanier]

do you know if folks at Mozilla is still working on XR?

I do not. Maybe @jdashg knows?

[Manishearth]

I don't think so. But unsure.

[kdashg]

It's roughly-speaking in maintenance mode: No one is actively working on it, but neither is it removed.

[marcoscaceres]

Ok, thanks @jdashg! I might poke at it, and see if I can get it to pick up the permission though the API.

[marcoscaceres]

In WebKit, I think this is called "xr-spatial-tracking"? https://immersive-web.github.io/webxr/#permissions-policy

[cabanier]

Yes, this is also the case in Chromium: https://github.com/chromium/chromium/blob/d7da0240cae77824d1eda25745c4022757499131/third_party/blink/renderer/core/permissions_policy/permissions_policy_features.json5#L295

[marcoscaceres]

Gecko has it in the experimental set:
https://searchfox.org/mozilla-central/source/dom/security/featurepolicy/FeaturePolicyUtils.cpp#58

Given that it's across all three, I'm merging.

[marcoscaceres]

Tested in web-platform-tests/wpt#30189