I want to do this so that I can get rid of Either<>. The reasons for getting rid
of either are multiple:
* It doesn't generate as nice C++ code using cbindgen.
* It isn't that nice to use either from Rust.
* cbindgen has bugs with zero-sized types.
I started using this for ColorOrAuto and a few others, for now.
Differential Revision: https://phabricator.services.mozilla.com/D19844
The only reason it was on style_traits is so that they could use it from some
other crates, but Servo eventually ends up getting the value from an integer, so
may as well pass it around and do that in the end of the process anyway.
Differential Revision: https://phabricator.services.mozilla.com/D16557
This is the first step to unprefix user-select.
This has no behavior change, it's just a nicer way to do the same thing which
allows us to unship individual values more easily using parse(condition).
Differential Revision: https://phabricator.services.mozilla.com/D11580
