DevTools: migrate the closeButton icons over to UI.Icon

third_party/WebKit/Source/devtools/front_end/ui/closeButton.css

 /*
  * Copyright (c) 2015 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.
  */
 
-div {
-    background-image: url(Images/toolbarButtonGlyphs.png);
-    background-size: 352px 168px;
-}
-
-@media (-webkit-min-device-pixel-ratio: 1.1) {
-div {
-    background-image: url(Images/toolbarButtonGlyphs_2x.png);
-}
-} /* media */
-
 .close-button {
     width: 14px;
     height: 14px;
-    background-position: -128px -96px;
     cursor: default;
+    display: flex;
+    align-items: center;
+    justify-content: center;
 }
 
-.close-button:hover {
-    background-position: -96px -96px;
+.hover-icon, .active-icon {
+    display: none;
 }
 
-.close-button:active {
-    background-position: -111px -96px;
+.close-button:hover .default-icon, .close-button:active .default-icon {
+    display: none;
 }
 
-.close-button-gray {
-    width: 13px;
-    height: 13px;
-    background-position: -175px -96px;
+.close-button:hover .hover-icon {
+    display: block;
 }
 
-.close-button-gray:hover {
-    background-position: -143px -96px;
+.close-button:active .hover-icon {
+    display: none !important;
 }
 
-.close-button-gray:active {
-    background-position: -160px -96px;
+.close-button:active .active-icon {
+    display: block;
 }