And remove some of the ::placeholder and ::cue hacks where we need to use
!important to make the property not apply for content but apply on UA sheets.
The comment about the white-space property was wrong, we don't enforce it with
!important in the UA stylesheets for <input> (we do for <textarea> though), so
I've kept the flag since it really applies.
Differential Revision: https://phabricator.services.mozilla.com/D37717
Now if you add a new inherited, pref-controlled property, you must
declare whether it can have an effect on scrollbar styles. If no,
then the property will be skipped in the assertions that check
whether our cached styles are equal to those we would compute.
Differential Revision: https://phabricator.services.mozilla.com/D37507
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
