style: Honor currentColor in HCM

I think it's fair to honor it, just like we honor system color keywords.

By definition if we're forcing colors currentColor can only be forced or
a system color.

Differential Revision: https://phabricator.services.mozilla.com/D157675

components/style/values/specified/color.rs

@@ -524,7 +524,7 @@ impl Color {
         match *self {
             #[cfg(feature = "gecko")]
             Color::InheritFromBodyQuirk => false,
-            Color::CurrentColor => false,
+            Color::CurrentColor => true,
             #[cfg(feature = "gecko")]
             Color::System(..) => true,
             Color::Numeric { ref parsed, .. } => allow_transparent && parsed.alpha == 0,