[w3c/payment-handler] fix: use sequence, not FrozenArray (#372)

closes #???

The following tasks have been completed:

 * [ ] web platform tests (link)
 * [ ] MDN Docs added (link)

Implementation commitment:

 * [ ] Safari (link to issue)
 * [ ] Chrome (link to issue)
 * [ ] Firefox (link to issue)
 * [ ] Edge (public signal)


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### :boom: Error: 500 Internal Server Error :boom: ###

[PR Preview](https://github.com/tobie/pr-preview#pr-preview) failed to build. _(Last tried on Jul 9, 2020, 4:40 AM UTC)_.

<details>
<summary>More</summary>


PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

:rotating_light: [Spec Generator](https://www.w3.org/2015/labs/) - Spec Generator is the web service used to build specs that rely on ReSpec.

:link: [Related URL](https://labs.w3.org/spec-generator/?type=respec&url=https%3A%2F%2Frawcdn.githack.com%2Fw3c%2Fpayment-handler%2Fba9ba4eb5aadec2dde1bc49ffb14e383b472bbc7%2Findex.html%3FisPreview%3Dtrue)

```
socket hang up
```

_If you don't have enough information above to solve the error by yourself (or to understand to which web service the error is related to, if any), please [file an issue](https://github.com/tobie/pr-preview/issues/new?title=Error%20not%20surfaced%20properly&body=See%20w3c/payment-handler%23372.)._
</details>

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-handler/pull/372

-- Commit Summary --

  * fix: use sequence, not FrozenArray

-- File Changes --

    M index.html (6)

-- Patch Links --

https://github.com/w3c/payment-handler/pull/372.patch
https://github.com/w3c/payment-handler/pull/372.diff

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