WebUI: Undo some usages of ES6 features that break uglify.

ui/webui/resources/cr_elements/cr_slider/cr_slider.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview
  * cr-slider wraps a paper-slider. It maps the slider's values from a linear UI
  * range to a range of real values.  When |value| does not map exactly to a
  * tick mark, it interpolates to the nearest tick.
  *
@@ skipping 38 matching lines @@
       this.value = this.tickValues[this.$.slider.immediateValue];
   },
 
   /**
    * Updates the knob position when |value| changes. If the knob is still being
    * dragged, this instead forces |value| back to the current position.
    * @private
    */
   valueChanged_: function() {
     // First update the slider settings if |tickValues| was set.
-    let numTicks = Math.max(1, this.tickValues.length);
+    var numTicks = Math.max(1, this.tickValues.length);
     this.$.slider.max = numTicks - 1;
     // Limit the number of ticks to 10 to keep the slider from looking too busy.
     /** @const */ var MAX_TICKS = 10;
     this.$.slider.snaps = numTicks < MAX_TICKS;
     this.$.slider.maxMarkers = numTicks < MAX_TICKS ? numTicks : 0;
 
     if (this.$.slider.dragging && this.tickValues.length > 0 &&
         this.value != this.tickValues[this.$.slider.immediateValue]) {
       // The value changed outside cr-slider but we're still holding the knob,
       // so set the value back to where the knob was.
@@ skipping 30 matching lines @@
       if (difference < minDifference) {
         closestIndex = i;
         minDifference = difference;
       }
     }
 
     assert(typeof closestIndex != 'undefined');
     return closestIndex;
   },
 });