Introduce DEFINE_DOCUMENT_TYPE_CASTS, and use it

Source/core/html/PluginDocument.h

Index: Source/core/html/PluginDocument.h
diff --git a/Source/core/html/PluginDocument.h b/Source/core/html/PluginDocument.h
index 9764a9dbedfdd5eb0492732840001ec56ed370a3..590cb30303ff9aeebb2cf4f5cb706e5e7581296e 100644
--- a/Source/core/html/PluginDocument.h
+++ b/Source/core/html/PluginDocument.h
@@ -61,26 +61,7 @@ private:
     RefPtr<Node> m_pluginNode;
 };
 
-inline PluginDocument* toPluginDocument(Document* document)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(!document || document->isPluginDocument());
-    return static_cast<PluginDocument*>(document);
-}
-
-inline const PluginDocument* toPluginDocument(const Document* document)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(!document || document->isPluginDocument());
-    return static_cast<const PluginDocument*>(document);
-}
-
-inline PluginDocument& toPluginDocument(Document& document)
-{
-    ASSERT_WITH_SECURITY_IMPLICATION(document.isPluginDocument());
-    return static_cast<PluginDocument&>(document);
-}
-
-// This will catch anyone doing an unnecessary cast.
-void toPluginDocument(const PluginDocument*);
+DEFINE_DOCUMENT_TYPE_CASTS(PluginDocument);
 
 }