Make fetch happen
<!-- Please describe your changes on the following line: -->
Moves XHR over to the fetch backend.
Previous PR: https://github.com/servo/servo/pull/114
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors (Will fix later)
<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11556)
<!-- Reviewable:end -->
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.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11221)
<!-- Reviewable:end -->
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
Fixed conflict
Fixed merge issue
Finished implementation
Disable tinyfiledialogs on Windows
addressed comments
Use ancestor's SubpageId
Move display alert from method to function
Add extra test for nested iframes
Addressed comments
Updated tinyfiledialogs
Added #[allow(unused_extern_crates)] to silence false positives
* bitflags, lazy_static and matches because macro_use
* alloc_jemalloc because builtin crate
See https://github.com/rust-lang/rust/issues/30849