slider: Don't hardcode scroll interval for smooth scrolling

Use the SLIDER_SCROLL_STEP constant instead for consistency.

https://bugzilla.gnome.org/show_bug.cgi?id=742648

js/ui/slider.js

@@ -197,7 +197,7 @@ const Slider = new Lang.Class({
             let [dx, dy] = event.get_scroll_delta();
             // Even though the slider is horizontal, use dy to match
             // the UP/DOWN above.
-            delta = -dy / 10;
+            delta = -dy * SLIDER_SCROLL_STEP;
         }
 
         this._value = Math.min(Math.max(0, this._value + delta), 1);