Add a command line option to disable native titlebar

ports/glutin/window.rs

@@ -87,6 +87,7 @@ impl Window {
                parent: glutin::WindowID) -> Rc<Window> {
         let mut glutin_window = glutin::WindowBuilder::new()
                             .with_title("Servo".to_string())
+                            .with_decorations(!opts::get().no_native_titlebar)
                             .with_dimensions(window_size.to_untyped().width, window_size.to_untyped().height)
                             .with_gl(Window::gl_version())
                             .with_visibility(is_foreground)