DevTools: Remove unused variables. Disallow unused variables with eslint

third_party/WebKit/Source/devtools/front_end/ui/ListWidget.js

 // Copyright 2015 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.
 /**
  * @unrestricted
  */
 UI.ListWidget = class extends UI.VBox {
   /**
    * @param {!UI.ListWidget.Delegate} delegate
    */
@@ skipping 96 matching lines @@
     this._updatePlaceholder();
   }
 
   /**
    * @param {*} item
    * @param {!Element} element
    * @return {!Element}
    */
   _createControls(item, element) {
     var controls = createElementWithClass('div', 'controls-container fill');
-    var gradient = controls.createChild('div', 'controls-gradient');
+    controls.createChild('div', 'controls-gradient');
     var buttons = controls.createChild('div', 'controls-buttons');
 
     var editButton = buttons.createChild('div', 'edit-button');
     editButton.title = Common.UIString('Edit');
     editButton.addEventListener('click', onEditClicked.bind(this), false);
 
     var removeButton = buttons.createChild('div', 'remove-button');
     removeButton.title = Common.UIString('Remove');
     removeButton.addEventListener('click', onRemoveClicked.bind(this), false);
 
@@ skipping 271 matching lines @@
 
   _cancelClicked() {
     var cancel = this._cancel;
     this._commit = null;
     this._cancel = null;
     this._item = null;
     this._index = -1;
     cancel();
   }
 };