style: Include anon boxes in CSSPseudoElementType, to remove ComputedStyle::mPseudoTag.

This is more consistent with what the Rust bits of the style system do, and
removes a pointer from ComputedStyle which is always nice.

This also aligns the Rust bits with the C++ bits re. not treating xul pseudos as
anonymous boxes. See the comment in nsTreeStyleCache.cpp regarding those.

Can't wait for XUL trees to die.

Differential Revision: https://phabricator.services.mozilla.com/D19002

components/style/properties/cascade.rs

@@ -236,7 +236,7 @@ where
                 parent_style.unwrap(),
                 parent_style_ignoring_first_line.unwrap()
             ) ||
-            parent_style.unwrap().pseudo() == Some(PseudoElement::FirstLine)
+            parent_style.unwrap().is_first_line_style()
     );
 
     let inherited_style = parent_style.unwrap_or(device.default_computed_values());