Auto merge of #16477 - sadmansk:url_param_browser, r=paulrouget

Pass URL to Browser::new(), delegate url checking logic to 3rd party

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1. Move the logic of computing the initial url from `opts.rs` to `/ports/servo/main.rs`
2. Add a `ServoUrl` argument to `Browser::new`

Based on the requested changes by @paulrouget:
>We can read the pref in main() instead. shell.homepage would be used if the url is not passed as an argument. I'm trying to decouple the "app" logic and the "web engine" logic. I think it's up to the app to set the initial URL, and I'm not sure the initial url should be part of opts.

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- [x] These changes fix #15636

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- [ ] There are tests for these changes

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