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

Source/core/dom/ElementRareData.cpp

 /*
  * Copyright (C) 2012 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 25 matching lines @@
 #include "core/rendering/style/RenderStyle.h"
 
 namespace WebCore {
 
 struct SameSizeAsElementRareData : NodeRareData {
     short indices[2];
     unsigned bitfields;
     RegionOversetState regionOversetState;
     LayoutSize sizeForResizing;
     IntSize scrollOffset;
-    void* pointers[9];
+    void* pointers[10];
 };
 
 COMPILE_ASSERT(sizeof(ElementRareData) == sizeof(SameSizeAsElementRareData), ElementRareDataShouldStaySmall);
 
 void ElementRareData::reportMemoryUsage(MemoryObjectInfo* memoryObjectInfo) const
 {
     MemoryClassInfo info(memoryObjectInfo, this, WebCoreMemoryTypes::DOM);
     NodeRareData::reportMemoryUsage(memoryObjectInfo);
 
     info.addMember(m_computedStyle, "computedStyle");
     info.addMember(m_dataset, "dataset");
     info.addMember(m_classList, "classList");
     info.addMember(m_shadow, "shadow");
     info.addMember(m_attributeMap, "attributeMap");
     info.addMember(m_generatedBefore, "generatedBefore");
     info.addMember(m_generatedAfter, "generatedAfter");
 }
 
 } // namespace WebCore