Introduce DEFINE_ELEMENT_DATA_TYPE_CASTS, and use it

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 02cc1eeb642396e3c642d3dca102972a857b3fdf..a0d5240a742def0a37700704d5cae280f94b9291 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -3117,7 +3117,7 @@ void Element::cloneAttributesFromElement(const Element& other)
     if (other.m_elementData->isUnique()
         && !ownerDocumentsHaveDifferentCaseSensitivity
         && !other.m_elementData->presentationAttributeStyle())
-        const_cast<Element&>(other).m_elementData = static_cast<const UniqueElementData*>(other.m_elementData.get())->makeShareableCopy();
+        const_cast<Element&>(other).m_elementData = toUniqueElementData(other.m_elementData)->makeShareableCopy();
 
     if (!other.m_elementData->isUnique() && !ownerDocumentsHaveDifferentCaseSensitivity && !needsURLResolutionForInlineStyle(other, other.document(), document()))
         m_elementData = other.m_elementData;
@@ -3142,7 +3142,7 @@ void Element::createUniqueElementData()
         m_elementData = UniqueElementData::create();
     else {
         ASSERT(!m_elementData->isUnique());
-        m_elementData = static_cast<ShareableElementData*>(m_elementData.get())->makeUniqueCopy();
+        m_elementData = toShareableElementData(m_elementData)->makeUniqueCopy();
     }
 }