Bug 1357357 - Make the parser of transition-property match the spec.

1. We add a new arm to TransitionProperty, TransitionProperty::Unsupported,
   which contains an Atom, so it's better to remove the Copy trait from
   TransitionProperty.
2. TransitionProperty::Unsupported(Atom) represents any non-animatable, custom,
   or unrecognized property, and we use Atom to store the ident string for
   serialization.

components/style/build_gecko.rs

@@ -659,6 +659,7 @@ mod bindings {
             "StyleBasicShape",
             "StyleBasicShapeType",
             "StyleShapeSource",
+            "StyleTransition",
             "nsCSSFontFaceRule",
             "nsCSSKeyword",
             "nsCSSPropertyID",