Update some feature gates.

CC #5286.

ports/glutin/lib.rs

@@ -5,7 +5,6 @@
 //! A simple application that uses glutin to open a window for Servo to display in.
 
 #![feature(int_uint)]
-#![feature(core)]
 #![feature(box_syntax)]
 
 #[macro_use] extern crate bitflags;

ports/glutin/window.rs

@@ -447,7 +447,7 @@ impl WindowMethods for Window {
 
     fn set_page_title(&self, title: Option<String>) {
         let title = match title {
-            Some(ref title) if title.len() > 0 => title.as_slice(),
+            Some(ref title) if title.len() > 0 => &**title,
             _ => "untitled",
         };
         let title = format!("{} - Servo", title);