[Feature Policy] Validate the plan for dependent policy-controlled features

[ddorwin]

One of the outcomes of #308 (see discussion starting at #308 (comment)) was a plan to have sub-features - policy-controlled features that depend on the main "xr" policy-controlled feature. As noted in that comment, this appears to be unique.

Before we commit to this plan, we should:

1. Ensure that such dependent policy-controlled features are acceptable from a Feature Policy and web platform point of view.
2. Figure out how the sub-feature mechanics would be specified.

The following are more details on (2).

Note that each policy-controlled feature has a state, and there isn't really a mechanism for checking other features. Is it sufficient that "xr" would make navigator.xr inaccessible or would the WebXR algorithms need to check combinations at each feature?

Does text related to the default allowlist remain unchanged? Specifically, text like the following:

The default allowlist for this feature is ["self"].

/cc @clelland

[probot-label]

This issue is fixed by PR #842