update prerender ClientHints-related algo

[Clqsin45]

The "Client Hints Infrastructure" specification updates the logic handling CH Cache[1], so this PR updates the logic on the prerender2 side accordingly to be consistent with the CH specification.

Including:

1. processing a request:

• Before: the processed hintSet is the union of prerender navigable's accept-CH cache and the global accept-CH cache.
• After: prerender navigable's accept-ch cache will override the global one.

2. update prerender navigable's accept-ch cache:

• Before: prerender navigable's accept-ch cache was set to the union of global accept-CH cache and the tokens specified by the 'Accept-CH' header.
• After: prerender navigable's accept-ch cache will be set to the tokens specified by the 'Accept-CH' header, because they are the newest.

[1] WICG/client-hints-infrastructure#135

[domenic]

Great work; thank you! I'll wait to merge this until after #237 lands so we can have a series of green builds on the main branch :)

[github-actions]

Preview:

• prerendering.html (diff base)

[Clqsin45]

Thank you for reviewing!! :D