[Web Install] Cross-origin case needs an install_url

[jyasskin]

https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/WebInstall/explainer_cross_domain.md#signatures-of-the-install-method-cross-origin has

In the absence of an install_url, the value defaults to that of the manifest's start_url.

But the other parameter is a manifest_id, not a URL from which a manifest could be fetched, so I don't think it works to omit the install_url. You also have "navigator.install(<manifest_id>[, <install_url>...])" a bit above that which also ought to change to make the install_url required.

[amandabaker]

Thanks @jyasskin for catching this! I think we just missed it when splitting this into 2 explainers, so I removed that line in #869.

If no install_url is provided, then the current document would be used as the install url, which would make it a same-origin install. Then if the provided manifest_id were for another origin (i.e. if you were trying to do a cross-origin install without an install_url) then installation would fail because the provided id does not have the same origin as the current document.