Implement typed array support for XMLHttpRequest's send API

[jdm]

Now that #20267 has merged, we can uncomment the BufferSource argument to allow XMLHttpRequest.send() to support sending typed arrays as the XHR body. This will require extracting the data from the typed array in this code.

Code: components/script/dom/webidls/XMLHttpRequest.webidl, components/script/dom/xmlhttprequest.rs
Tests: ./mach test-wpt tests/wpt/web-platform-tests/xhr/ (this should generate a bunch of new test pass results)

[nupurbaghel]

This appears to be an interesting issue. I would surely like to work on it 😄

[nupurbaghel]

The BodyInit requires extracting both the data and content_type. For a typed array, I was trying to get data using to_vec() method which requires some dereferencing I think, but I was wondering how the content type should be defined here. Should we give it a value None if its absent and Int32 otherwise ?

[jdm]

https://fetch.spec.whatwg.org/#concept-bodyinit-extract does not assign a value for content_type when the input is an arraybuffer, so leaving a None value is appropriate.

[nupurbaghel]

I tried using the following code to extract data

Some(DocumentOrBodyInit::ArrayBuffer(ref typedarray)) => Some((typedarray.to_vec(), None))

This results in an error: cannot borrow immutable borrowed content *typedarray as mutable.
I couldn't find a better match than the to_vec() method which requires a mutable reference of data. data() method of typed array was one other option but that is a private method.
I need some help on how to proceed 😅
Thank you so much :)

[jdm]

If you rebase against Servo master, that problem should be gone (it was fixed by servo/rust-mozjs#408).