Add toPluginDocument() convenience method that takes a reference in argument

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 458dfc1039cf05e4fce7f61fc9203bf69dd03689..9764a9dbedfdd5eb0492732840001ec56ed370a3 100644
--- a/Source/core/html/PluginDocument.h
+++ b/Source/core/html/PluginDocument.h
@@ -73,6 +73,12 @@ inline const PluginDocument* toPluginDocument(const Document* document)
     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*);