Introduce DEFINE_CUSTOM_FONT_DATA_TYPE_CASTS, and use it

Source/core/css/CSSValue.cpp

Index: Source/core/css/CSSValue.cpp
diff --git a/Source/core/css/CSSValue.cpp b/Source/core/css/CSSValue.cpp
index df426652acac916c97f4cad4e2af631a644fd849..851cefcf83e09466c4eebe13af88be3b340a64a5 100644
--- a/Source/core/css/CSSValue.cpp
+++ b/Source/core/css/CSSValue.cpp
@@ -408,7 +408,7 @@ void CSSValue::finalizeGarbageCollectedObject()
 {
     if (m_isTextClone) {
         ASSERT(isCSSOMSafe());
-        static_cast<TextCloneCSSValue*>(this)->~TextCloneCSSValue();
+        toTextCloneCSSValue(this)->~TextCloneCSSValue();
         return;
     }
     ASSERT(!isCSSOMSafe() || isSubtypeExposedToCSSOM());
@@ -512,7 +512,7 @@ void CSSValue::trace(Visitor* visitor)
 {
     if (m_isTextClone) {
         ASSERT(isCSSOMSafe());
-        static_cast<TextCloneCSSValue*>(this)->traceAfterDispatch(visitor);
+        toTextCloneCSSValue(this)->traceAfterDispatch(visitor);
         return;
     }
     ASSERT(!isCSSOMSafe() || isSubtypeExposedToCSSOM());