style: Add a ValueInfo trait for exposing types needed by devtools.

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

components/style/macros.rs

@@ -68,8 +68,9 @@ macro_rules! try_match_ident_ignore_ascii_case {
 macro_rules! define_keyword_type {
     ($name:ident, $css:expr) => {
         #[allow(missing_docs)]
-        #[derive(Animate, Clone, ComputeSquaredDistance, Copy, MallocSizeOf, PartialEq,
-                 ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
+        #[derive(Animate, Clone, ComputeSquaredDistance, Copy, MallocSizeOf,
+                 PartialEq, SpecifiedValueInfo, ToAnimatedValue, ToAnimatedZero,
+                 ToComputedValue, ToCss)]
         pub struct $name;
 
         impl fmt::Debug for $name {