ScreenShield: wake up the screen when resuming from suspend

At some point ScreenShield had code to do this, I don't know when
it was lost, but it makes sense and avoids having to move the mouse
just to see the shield.

https://bugzilla.gnome.org/show_bug.cgi?id=726378

js/ui/screenShield.js

@@ -729,7 +729,7 @@ const ScreenShield = new Lang.Class({
         } else {
             this._inhibitSuspend();
 
-            this._onUserBecameActive();
+            this._wakeUpScreen();
         }
     },