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

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

Index: ui/webui/resources/cr_elements/cr_slider/cr_slider.js
diff --git a/ui/webui/resources/cr_elements/cr_slider/cr_slider.js b/ui/webui/resources/cr_elements/cr_slider/cr_slider.js
index 0d4d6107c3465c37d6ade85b4ac5e554d8acbf57..128ddfaa3548b253622a9f63fcc59116dc00f70a 100644
--- a/ui/webui/resources/cr_elements/cr_slider/cr_slider.js
+++ b/ui/webui/resources/cr_elements/cr_slider/cr_slider.js
@@ -56,7 +56,7 @@ Polymer({
    */
   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;