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/values/specified/counters.rs

@@ -99,7 +99,8 @@ fn is_decimal(counter_type: &CounterStyleType) -> bool {
 /// The specified value for the `content` property.
 ///
 /// https://drafts.csswg.org/css-content/#propdef-content
-#[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, ToComputedValue, ToCss)]
+#[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, SpecifiedValueInfo,
+         ToComputedValue, ToCss)]
 pub enum Content {
     /// `normal` reserved keyword.
     Normal,
@@ -113,7 +114,8 @@ pub enum Content {
 }
 
 /// Items for the `content` property.
-#[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, ToComputedValue, ToCss)]
+#[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, SpecifiedValueInfo,
+         ToComputedValue, ToCss)]
 pub enum ContentItem {
     /// Literal string content.
     String(Box<str>),