Make sure we don't recreate boxes just because we have a ::first-line.

Servo part of part 5 of the fix for Gecko bug 1324619.  r=emilio

components/style/gecko/pseudo_element.rs

@@ -87,6 +87,12 @@ impl PseudoElement {
         *self == PseudoElement::FirstLetter
     }
 
+    /// Whether this pseudo-element is ::first-line.
+    #[inline]
+    pub fn is_first_line(&self) -> bool {
+        *self == PseudoElement::FirstLine
+    }
+
     /// Whether this pseudo-element is ::-moz-fieldset-content.
     #[inline]
     pub fn is_fieldset_content(&self) -> bool {