[whatwg/fetch] Skip unnecessary checks when we have a preloaded response candidate in main fetch (PR #1803)

This PR skips checks that are not needed when we have a preloaded response candidate in the [main fetch](https://fetch.spec.whatwg.org/#concept-main-fetch) algorithm. We talked about this offline at the last TPAC, but the main motivation behind this is to optimize reusing preloaded content where possible.

The main observable difference due to this change is that once a preload is made with a certain CSP policy, a subsequent request for the same resource will get the same response. This means that dynamically setting the CSP policy will not affect the ability to use preloaded content.

We could maybe argue the same for mixed content checks, but it seems better to respond to live changes in user preferences to block insecure content. Although, I'm open to thoughts and could see an argument either way.

This PR also skips all the irrelevant checks for clarity and ease of implementation, but another approach would be to only skip the observable checks in the spec.

@annevk @domenic Could you please take a look? Thank you!

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

  https://github.com/whatwg/fetch/pull/1803

-- Commit Summary --

  * Skip unnecessary checks when we have a preloaded response candidate in main fetch

-- File Changes --

    M fetch.bs (24)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1803.patch
https://github.com/whatwg/fetch/pull/1803.diff

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