DevTools: convert a few more icons to UI.Icon.

third_party/WebKit/Source/devtools/front_end/elements/stylesSectionTree.css

 /*
  * Copyright 2016 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.
  */
 
 .tree-outline {
     padding: 0;
 }
 
@@ skipping 161 matching lines @@
 .tree-outline li.child-editing .styles-clipboard-only {
     display: none;
 }
 
 /* Matched styles */
 
 :host-context(.matched-styles) .tree-outline li {
     margin-left: 0 !important;
 }
 
-:host-context(.matched-styles) .tree-outline li.parent .expand-element {
+.expand-icon {
     -webkit-user-select: none;
-    -webkit-mask-image: url(Images/toolbarButtonGlyphs.png);
-    -webkit-mask-size: 352px 168px;
+    background-color: #777;
+    margin-left: -6px;
     margin-right: 2px;
-    margin-left: -6px;
-    background-color: #777;
-    width: 8px;
-    height: 10px;
-    display: inline-block;
 }
 
-@media (-webkit-min-device-pixel-ratio: 1.1) {
-:host-context(.matched-styles) .tree-outline li.parent .expand-element {
-    -webkit-mask-image: url(Images/toolbarButtonGlyphs_2x.png);
-}
-} /* media */
-
-:host-context(.matched-styles) .tree-outline li.parent .expand-element {
-    -webkit-mask-position: -4px -96px;
-}
-
-:host-context(.matched-styles) .tree-outline li.parent.expanded .expand-element {
-    -webkit-mask-position: -20px -96px;
+.tree-outline li:not(.parent) .expand-icon {
+    display: none;
 }
 
 :host-context(.matched-styles:not(.read-only):hover) .enabled-button {
     visibility: visible;
 }
 
 :host-context(.matched-styles:not(.read-only)) .tree-outline li.disabled .enabled-button {
     visibility: visible;
 }
 
 :host-context(.matched-styles) ol.expanded {
     margin-left: 16px;
 }