Remove unused code (3/N)
<!-- Please describe your changes on the following line: -->
Third PR in a series of PRs to remove unused/dead code from servo, powered by an (upcoming) tool of mine. Please take a look and tell me if you want to keep something.
* First PR: #23477
* Second PR: #23498
---
<!-- 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
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)
<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they only remove dead code
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- 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="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23499)
<!-- Reviewable:end -->
Receive streams in WebRTC (and MediaStreamTrack support)
This adds the `ontrack` event handler to webrtc, and all the `MediaStreamTrack` stuff necessary to make it work.
WebRTC has the ability to group media tracks into streams using MSIDs, but I haven't yet figured out how to do this. For now, `ontrack` should work.
This _should_ be complete, but it hasn't yet been tested (hence the WIP)
r? @ferjm or @jdm
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23342)
<!-- Reviewable:end -->
MediaStream playback through audio and video elements
- [X] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] There are tests for these changes
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23157)
<!-- Reviewable:end -->
Update WebIDL.py
I'm working on getting [attributes on types landed in upstream webidl](https://bugzilla.mozilla.org/show_bug.cgi?id=1359269). In preparation for that, I'd like to update WebIDL.py and deal with all the conflicts first so that updating for that bug is easier.
(This PR doesn't need to be reviewed and landed right now, I can just roll it into the PR for implementing attributes on types, but I suspect it would be easier to land this first)
Probably should squash before landing, the steps are separated out for ease of review.
r? @nox @jdm
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22934)
<!-- Reviewable:end -->
