DevTools: make toolbar button click targets square

Source/devtools/front_end/sources/sourcesView.css

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 19 matching lines @@
 
 #sources-panel-sources-view {
     flex: auto;
     position: relative;
 }
 
 #sources-panel-sources-view .sources-toolbar {
     display: flex;
     position: relative;
     flex: 0 0 20px;
-    background-color: #eee;
+    background-color: #f3f3f3;
     border-top: 1px solid rgb(196,196,196);
     overflow: hidden;
 }
 
 .material #sources-panel-sources-view .sources-toolbar {
     flex: 0 0 24px;
 }
 
 #sources-panel-sources-view .sources-toolbar .toolbar {
     margin-top: -2px;
@@ skipping 68 matching lines @@
     to {
         background-color: inherit;
         color: inherit;
     }
 }
 
 .source-frame-value-update-highlight {
     -webkit-animation: source-frame-value-update-highlight-animation 0.8s 1 cubic-bezier(0, 0, 0.2, 1);
     border-radius: 2px;
 }