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

Source/core/dom/ElementRareData.h

Index: Source/core/dom/ElementRareData.h
diff --git a/Source/core/dom/ElementRareData.h b/Source/core/dom/ElementRareData.h
index 84f37adfe9c3e49922ba3bca92435ee9e0820e98..dc8a4e09e711d009abefd65ab97e375839b509d2 100644
--- a/Source/core/dom/ElementRareData.h
+++ b/Source/core/dom/ElementRareData.h
@@ -182,6 +182,7 @@ private:
 
     RefPtr<PseudoElement> m_generatedBefore;
     RefPtr<PseudoElement> m_generatedAfter;
+    RefPtr<PseudoElement> m_backdrop;
 
     ElementRareData(RenderObject*);
     void releasePseudoElement(PseudoElement*);
@@ -221,6 +222,7 @@ inline ElementRareData::~ElementRareData()
     ASSERT(!m_shadow);
     ASSERT(!m_generatedBefore);
     ASSERT(!m_generatedAfter);
+    ASSERT(!m_backdrop);
 }
 
 inline void ElementRareData::setPseudoElement(PseudoId pseudoId, PassRefPtr<PseudoElement> element)
@@ -234,6 +236,10 @@ inline void ElementRareData::setPseudoElement(PseudoId pseudoId, PassRefPtr<Pseu
         releasePseudoElement(m_generatedAfter.get());
         m_generatedAfter = element;
         break;
+    case BACKDROP:
+        releasePseudoElement(m_backdrop.get());
+        m_backdrop = element;
+        break;
     default:
         ASSERT_NOT_REACHED();
     }
@@ -246,6 +252,8 @@ inline PseudoElement* ElementRareData::pseudoElement(PseudoId pseudoId) const
         return m_generatedBefore.get();
     case AFTER:
         return m_generatedAfter.get();
+    case BACKDROP:
+        return m_backdrop.get();
     default:
         return 0;
     }