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This PR implements the fetch method, which is described in [the Fetch Spec](https://fetch.spec.whatwg.org/#fetch-method). The expected wpt results are updated as well.
A few comments about the current fetch implementation:
- The fetch method manually calls `JSAutoCompartment` in order to prevent SpiderMonkey from crashing. This may have to change in the future.
- Not all `FetchResponseListener` methods are implemented.
- `net_traits::CoreResourceMsg::Fetch` message takes a `net_traits::request::RequestInit`. However, when the fetch method is called, a `RequestBinding::RequestInit` is given. The fetch method constructs a `dom::request::Request` with the given `RequestInit`, then creates `net_traits::request::RequestInit` from the dom Request object for the fetch message.
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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#11898 (github issue number if applicable).
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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____
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Source-Repo: https://github.com/servo/servo
Source-Revision: 5996e008a34dd92602acb7bbd3ea41a880053110
