Auto merge of #21710 - mandreyel:load-url-privacy-mode-preservation, r=paulrouget

Fix private browsing mode preservation when loading URL

`Constellation::load_url` disregards the existing `BrowsingContext`'s `is_private` field when loading a new URL and creating a new `Pipeline` for the page load, which means that the `Pipeline` is always passed the public resource thread, even if its browsing context is in private browsing mode. This PR fixes that.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21706 (github issue number if applicable).

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- [x] I'm not actually sure. I haven't found tests related to private browsing so maybe that's why this bug wasn't caught earlier?

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