DevTools: make toolbar button click targets square

Source/devtools/front_end/elements/elementsTreeOutline.css

 /*
  * Copyright (c) 2014 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.
  */
 
 .elements-disclosure {
     width: 100%;
     display: inline-block;
     line-height: normal;
@@ skipping 53 matching lines @@
     background-color: rgba(56, 121, 217, 0.1);
     border-radius: 5px;
 }
 
 .elements-disclosure li.parent.expanded::before {
     -webkit-mask-position: -20px -96px;
 }
 
 .elements-disclosure li.selected .selection {
     display: block;
-    background-color: rgb(212, 212, 212);
+    background-color: #dadada;
 }
 
 .elements-disclosure ol {
     list-style-type: none;
     -webkit-padding-start: 12px;
     margin: 0;
 }
 
 .elements-disclosure ol.children {
     display: none;
@@ skipping 208 matching lines @@
 }
 
 ol:hover > li > .elements-tree-shortcut-link {
     display: initial;
 }
 
 .elements-tree-shortcut-link {
     color: rgb(87, 87, 87);
     display: none;
 }