[Performance Timeline API](https://www.w3.org/TR/performance-timeline-2/) implementation.
This API is required to allow DOM access to the [Paint Timing API](https://wicg.github.io/paint-timing/#example) metrics implemented in #17256. Unfortunately, I couldn't test it properly, as its usage depends on other APIs like the Paint Timing, User Timing, Resource Timing or Server Timing APIs. I'll work in the integration with the Paint Timing API next.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] There are [WPTs](https://github.com/servo/servo/tree/master/tests/wpt/web-platform-tests/performance-timeline) for this API, however they depend on the implementation of the User Timing and the Resource Timing APIs, which I'll hopefully be implementing soon.
Source-Repo: https://github.com/servo/servo
Source-Revision: 6988c7424ddfdd9a98e6a458ff2ad307a74237aa
f149cfd6f3