implement Body for dom::Response and dom::Request
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This PR adds dom::Response and dom::Request's `Text()`, `Blob()`, `Json()`, and (part of the) `FormData()` functions, part of the [Body mixin](https://fetch.spec.whatwg.org/#body). The corresponding tests have also been updated as passing.
@jeenalee also contributed to this PR, primarily the `Blob()`, `Json()`, and `FormData()` implementations.
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- [ ] These changes fix #__ (github issue number if applicable).
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Make fetch happen
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Moves XHR over to the fetch backend.
Previous PR: https://github.com/servo/servo/pull/114
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Add file backend support for Blob and related
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` does not report any errors
- [x] These changes fix#10851, related to #11131
- [x] These changes do not require tests because the implementation is partial and can't work alone
1. Add new backend to `Blob` and a `BlobImpl` struct to abstract multiple backends
2. Rewrite most interfaces of `Blob` to accommodate the change
3. Change the `read` behaviour of `FileReader`, considering the case when blob is file-backed and not cached
The design is still immature, welcome comments!
- [x] I used `DOMRefCell` to cache the bytes in `BlobImpl`, is it sound?
- [x] The interfaces (like `BlobImpl::get_bytes`) handle requests in a default-to-empty way when the inner `DataSlice` is not cached. It might be possible to handle this condition better.
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Changes include:
- Add BlobImpl to Blob, and related caching mechanism
- Expose ResourceThreads to document_loader, workerglobalscope, worker, and global
- Fix encode_multipart_form_data
- Other small fixes to accommodate the above changes
