Use cssparser's new_with_line_number_offset

cssparser provides a way to set the initial line number on a
ParserInput.  This patch changes servo to use this facility, rather than
reimplement the same functionality itself.

components/style/encoding_support.rs

@@ -72,7 +72,7 @@ impl Stylesheet {
                              stylesheet_loader,
                              error_reporter,
                              quirks_mode,
-                             0u64)
+                             0)
     }
 
     /// Updates an empty stylesheet with a set of bytes that reached over the