At-risk feature: tab_strip scope_patterns accepting a string

[mgiuca]

The scope_patterns member is parsed using URLPattern's build a URL pattern from an infra value, which accepts the URL pattern as either a string or a map containing URL pattern components.

Currently, no implementation accepts the string form. (As there is only one implementation, Chromium, and it does not currently support the string form. Chromium tracking bug: crbug/330640840).

We could change the spec to expect only a map, but it would be a shame, as the string form is much easier to use, and the URLPattern spec explicitly says that JSON-based usages should use this algorithm. So for now, keeping it in the spec but acknowledging it is at risk.

If this becomes supported, the usage example should be updated to use the more natural string syntax.