Match eager pseudos after the primary cascade.

This is necessary in order to make the computation of eager pseudos depend on
the primary ComputedValues, which we want to do for ::first-letter/::first-line.

This also fixes a bug where the behavior of EagerPseudoStyles::is_empty was reversed
in both the implementation and the callsite.

MozReview-Commit-ID: EXBxclyHWXu

components/style/data.rs

@@ -80,7 +80,7 @@ pub struct EagerPseudoStyles(Option<Box<[Option<ComputedStyle>]>>);
 impl EagerPseudoStyles {
     /// Returns whether there are any pseudo styles.
     pub fn is_empty(&self) -> bool {
-        self.0.is_some()
+        self.0.is_none()
     }
 
     /// Returns a reference to the style for a given eager pseudo, if it exists.