Introduce DEFINE_CONTENT_DATA_TYPE_CASTS, and use it

Source/core/rendering/style/RenderStyle.cpp

Index: Source/core/rendering/style/RenderStyle.cpp
diff --git a/Source/core/rendering/style/RenderStyle.cpp b/Source/core/rendering/style/RenderStyle.cpp
index 95baaad0f4520673dc06a3ecaa1095c2c6fd398e..d89e8f6753fdc0e58afeeaa2c7cc5dd4aad2201b 100644
--- a/Source/core/rendering/style/RenderStyle.cpp
+++ b/Source/core/rendering/style/RenderStyle.cpp
@@ -784,7 +784,7 @@ void RenderStyle::setContent(const String& string, bool add)
         if (lastContent) {
             // We attempt to merge with the last ContentData if possible.
             if (lastContent->isText()) {
-                TextContentData* textContent = static_cast<TextContentData*>(lastContent);
+                TextContentData* textContent = toTextContentData(lastContent);
                 textContent->setText(textContent->text() + string);
             } else
                 lastContent->setNext(ContentData::create(string));