Most of types just derive it using proc_macro directly. Some of value
types need manual impl.
In my current plan, this new trait will be used in bug 1434130 to expose
values as well.
Bug: 1455576
Reviewed-by: emilio
MozReview-Commit-ID: LI7fy45VkRw
This should fix the following two "expected to fail" tests:
- getComputedStyle(elem) for url() listStyleImage uses the resolved URL
and elem.style uses the original URL
- getComputedStyle(elem) for url() listStyle uses the resolved URL
and elem.style uses the original URL