ByteString isn't supported in a union

[jdm]

Blocks #11897.

malisa@malisa-ThinkPad-T450s:~/git_repos/malisa-servo/servo$ ./mach build -d
   Compiling script v0.0.1 (file:///home/malisa/git_repos/malisa-servo/servo/components/script)
/home/malisa/git_repos/malisa-servo/servo/target/debug/build/script-daadca0f01f72a7e/out/UnionTypes.rs:840:32: 840:51 error: wrong number of type arguments: expected 1, found 0 [E0243]
/home/malisa/git_repos/malisa-servo/servo/target/debug/build/script-daadca0f01f72a7e/out/UnionTypes.rs:840     ByteStringSequenceSequence(Vec</*ByteString*/>),
                                                                                                                                          ^~~~~~~~~~~~~~~~~~~
/home/malisa/git_repos/malisa-servo/servo/components/script/dom/bindings/mod.rs:176:9: 176:62 note: in this expansion of include!
/home/malisa/git_repos/malisa-servo/servo/target/debug/build/script-daadca0f01f72a7e/out/UnionTypes.rs:840:32: 840:51 help: run `rustc --explain E0243` to see a detailed explanation
/home/malisa/git_repos/malisa-servo/servo/target/debug/build/script-daadca0f01f72a7e/out/UnionTypes.rs:886:111: 886:130 error: wrong number of type arguments: expected 1, found 0 [E0243]
/home/malisa/git_repos/malisa-servo/servo/target/debug/build/script-daadca0f01f72a7e/out/UnionTypes.rs:886     unsafe fn TryConvertToByteStringSequenceSequence(cx: *mut JSContext, value: HandleValue) -> Result<Option<Vec</*ByteString*/>>, ()> {
                                                                                                                                                                                                                         ^~~~~~~~~~~~~~~~~~~
/home/malisa/git_repos/malisa-servo/servo/components/script/dom/bindings/mod.rs:176:9: 176:62 note: in this expansion of include!
/home/malisa/git_repos/malisa-servo/servo/target/debug/build/script-daadca0f01f72a7e/out/UnionTypes.rs:886:111: 886:130 help: run `rustc --explain E0243` to see a detailed explanation
error: aborting due to 2 previous errors
error: Could not compile `script`.

To learn more, run the command again with --verbose.
Build completed in 0:00:17

[jdm]

It goes into the else codepath in getUnionTypeTemplateVars that returns the commented-out type name. This should be a straightforward fix to add a branch like the USVString one. We should add a use of this to TestBinding.webidl and testbinding.rs to verify that it does the right thing.

Code: components/script/dom/bindings/codegen/CodegenRust.py

If the changes to TestBinding.webidl and testbinding.rs compile, that's good enough for a PR.

[highfive]

cc @wafflespeanut

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[malisas]

I'll work on this.