Devtools UI: Add high DPI chevron overflow icon

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 341 matching lines @@
     height: 20px;
     transform: scale(0.8) rotate(90deg);
     -webkit-mask-image: url(Images/toolbarButtonGlyphs.png);
     -webkit-mask-size: 352px 168px;
     background-color: #FFFFFF;
     z-index: 1;
     margin-top: -3px;
     cursor: pointer;
 }
 
+@media (-webkit-min-device-pixel-ratio: 1.5) {
+.node-actions-toggle {
+    -webkit-mask-image: url(Images/toolbarButtonGlyphs_2x.png);
+}
+} /* media */
+
 .gutter-container.has-decorations .node-actions-toggle {
     opacity: 0;
 }
 
 .node-actions-container.expanded > .node-actions-toolbar {
     visibility: visible;
 }
 
 .node-actions-toolbar-below {
     margin-top: 2px;
 }