Devtools UI: Add high DPI chevron overflow icon

Source/devtools/front_end/ui/tabbedPane.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  * Copyright (C) 2011 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:
  *
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 137 matching lines @@
 .tabbed-pane-header-tabs-drop-down-container > .glyph {
     height: 26px;
     background-color: hsla(0,0%,20%,1);
     -webkit-mask-image: url(Images/toolbarButtonGlyphs.png);
     -webkit-mask-position: -68px -143px;
     -webkit-mask-size: 352px 168px;
     width: 24px;
 }
 
 @media (-webkit-min-device-pixel-ratio: 1.5) {
-.tabbed-pane-header-tabs-drop-down-container {
+.tabbed-pane-header-tabs-drop-down-container > .glyph {
     -webkit-mask-image: url(Images/toolbarButtonGlyphs_2x.png);
 }
 } /* media */
 
 .tabbed-pane-header-tabs-drop-down-container:hover {
     background-color: rgb(229, 229, 229);
 }
 
 .tabbed-pane-header-tabs-drop-down-container.measuring {
     visibility: hidden;
@@ skipping 81 matching lines @@
     }
 }
 
 .tabbed-pane-tab-slider.enabled {
     visibility: visible;
 }
 
 .tabbed-pane-header.tabbed-pane-no-header-background {
     background-color: transparent;
 }