Add string needed if we implement :nth-*() pseudo-classes. b=75375 r+sr=bzbarsky

dom/locales/en-US/chrome/layout/css.properties

@@ -97,6 +97,7 @@ PENegationNoClose=Missing closing ')' in negation pseudo-class '%1$S'.
 PENegationBadArg=Missing argument in negation pseudo-class '%1$S'.
 PEPseudoClassArgEOF=argument to pseudo-class selector
 PEPseudoClassArgNotIdent=Expected identifier for pseudo-class parameter but found '%1$S'.
+PEPseudoClassArgNotNth=Expected part of argument to pseudo-class but found '%1$S'.
 PEPseudoClassNoClose=Missing closing ')' in pseudo-class, found '%1$S' instead.
 PEPseudoClassNoArg=Missing argument in pseudo-class '%1$S'.
 PESelectorEOF=selector