overviewControls: Fix initial slide-in animation

Controls are slided in by animating translationX from the actor's
width to 0; however as _updateTranslation() will skip the animation
when the property is already at its target value and 0 happens to be
the initial value of translationX, the initial animation is skipped.
Fix this by initializing translationX to undefined, which will always
differ from a valid target value.

js/ui/overviewControls.js

@@ -36,7 +36,7 @@ const SlideLayout = new Lang.Class({
 
     _init: function(params) {
         this._slideX = 1;
-        this._translationX = 0;
+        this._translationX = undefined;
         this._direction = SlideDirection.LEFT;
 
         this.parent(params);