DevTools: [SSP] implement "add rule to end" of arbitrary style sheet.

Source/devtools/front_end/sidebarPane.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  *
  * 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
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 145 matching lines @@
     background-position: -23px 0;
 }
 
 .sidebar-pane-toolbar > .pane-title-button:active,
 .sidebar-pane-toolbar > .pane-title-button.toggled {
     background-position: -46px 0;
 }
 
 .sidebar-pane-toolbar > .pane-title-button.add {
     background-image: url(Images/paneAddButtons.png);
+    position: relative;
+}
+
+.sidebar-pane-toolbar > .pane-title-button.add .long-click-glyph {
+    background-color: rgba(135, 135, 135, 1);
+    -webkit-mask-image: url(Images/statusbarButtonGlyphs.png);
+    -webkit-mask-position: -298px -53px;
+    -webkit-mask-size: 320px 144px;
 }
 
 .sidebar-pane-toolbar > .pane-title-button.element-state {
     background-image: url(Images/paneElementStateButtons.png);
 }
 
 .sidebar-pane-toolbar > .pane-title-button.refresh {
     background-image: url(Images/paneRefreshButtons.png);
 }
 
@@ skipping 31 matching lines @@
     visibility: hidden;
 }
 
 .styles-section.matched-styles:not(.read-only):hover .sidebar-pane-button-new-rule {
     visibility: visible;
 }
 
 .sidebar-pane-button-new-rule:hover {
     background-position: -23px 0;
 }