Introduce DEFINE_CONTENT_DATA_TYPE_CASTS, and use it

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index 0ff0c03f9415e195f9206afcadfed11c57a22acd..d34ce94922e4e15fcff7e6cb129cc3f92f423e0b 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -1346,15 +1346,16 @@ static PassRefPtrWillBeRawPtr<CSSValue> valueForContentData(const RenderStyle& s
     RefPtrWillBeRawPtr<CSSValueList> list = CSSValueList::createSpaceSeparated();
     for (const ContentData* contentData = style.contentData(); contentData; contentData = contentData->next()) {
         if (contentData->isCounter()) {
-            const CounterContent* counter = static_cast<const CounterContentData*>(contentData)->counter();
+            const CounterContent* counter = toCounterContentData(contentData)->counter();
             ASSERT(counter);
             list->append(cssValuePool().createValue(counter->identifier(), CSSPrimitiveValue::CSS_COUNTER_NAME));
         } else if (contentData->isImage()) {
-            const StyleImage* image = static_cast<const ImageContentData*>(contentData)->image();
+            const StyleImage* image = toImageContentData(contentData)->image();
             ASSERT(image);
             list->append(image->cssValue());
-        } else if (contentData->isText())
-            list->append(cssValuePool().createValue(static_cast<const TextContentData*>(contentData)->text(), CSSPrimitiveValue::CSS_STRING));
+        } else if (contentData->isText()) {
+            list->append(cssValuePool().createValue(toTextContentData(contentData)->text(), CSSPrimitiveValue::CSS_STRING));
+        }
     }
     return list.release();
 }