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 {

components/style/matching.rs

@@ -827,7 +827,7 @@ pub trait MatchMethods : TElement {
             return StyleDifference::new(RestyleDamage::empty(), StyleChange::Unchanged)
         }
 
-        if pseudo.map_or(false, |p| p.is_first_letter()) {
+        if pseudo.map_or(false, |p| p.is_first_letter() || p.is_first_line()) {
             // No one cares about this pseudo, and we've checked above that
             // we're not switching from a "cares" to a "doesn't care" state
             // or vice versa.

components/style/servo/selector_parser.rs

@@ -42,6 +42,8 @@ pub enum PseudoElement {
     Selection,
     // If/when :first-letter is added, update is_first_letter accordingly.
 
+    // If/when :first-line is added, update is_first_line accordingly.
+
     // If/when ::first-letter, ::first-line, or ::placeholder are added, adjust
     // our property_restriction implementation to do property filtering for
     // them.  Also, make sure the UA sheet has the !important rules some of the
@@ -125,6 +127,12 @@ impl PseudoElement {
         false
     }
 
+    /// Whether the current pseudo element is :first-line
+    #[inline]
+    pub fn is_first_line(&self) -> bool {
+        false
+    }
+
     /// Whether this pseudo-element is eagerly-cascaded.
     #[inline]
     pub fn is_eager(&self) -> bool {