Didn't touch mozjs or rust-mozjs because implementing that in the code generator didn't seem too easy. I'm using the same workaround that the TextDecoder does.
Using the OsRng should be the right choice here? As the OS keeps state for us we wouldn't need to have a global rng instance to keep around.
Fixes#4666.
Source-Repo: https://github.com/servo/servo
Source-Revision: c0222628264423a67bf98775be83dcf2f85211ab
