Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/rendering/RenderTheme.cpp

 /**
  * This file is part of the theme implementation for form controls in WebCore.
  *
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 477 matching lines @@
 
         if (RuntimeEnabledFeatures::inputTypeColorEnabled()) {
             runtimeCSS.appendLiteral("input[type=\"color\"][list] { -webkit-appearance: menulist; width: 88px; height: 23px;}");
             runtimeCSS.appendLiteral("input[type=\"color\"][list]::-webkit-color-swatch-wrapper { padding-left: 8px; padding-right: 24px;}");
             runtimeCSS.appendLiteral("input[type=\"color\"][list]::-webkit-color-swatch { border-color: #000000;}");
         }
     }
     if (RuntimeEnabledFeatures::dialogElementEnabled()) {
         runtimeCSS.appendLiteral("dialog:not([open]) { display: none; }");
         runtimeCSS.appendLiteral("dialog { position: absolute; left: 0; right: 0; margin: auto; border: solid; padding: 1em; background: white; color: black;}");
+        runtimeCSS.appendLiteral("dialog::backdrop { background: rgba(0,0,0,0.1); }");
     }
 
     return runtimeCSS.toString();
 }
 
 String RenderTheme::formatMediaControlsTime(float time) const
 {
     if (!std::isfinite(time))
         time = 0;
     int seconds = (int)fabsf(time);
@@ skipping 871 matching lines @@
 
     // padding - not honored by WinIE, needs to be removed.
     style->resetPadding();
 
     // border - honored by WinIE, but looks terrible (just paints in the control box and turns off the Windows XP theme)
     // for now, we will not honor it.
     style->resetBorder();
 }
 
 } // namespace WebCore