Add missing null check in PageSerializer::serializeCSSStyleSheet()

Source/core/page/PageSerializer.h

Index: Source/core/page/PageSerializer.h
diff --git a/Source/core/page/PageSerializer.h b/Source/core/page/PageSerializer.h
index 6c390d569b7f130398628d87a0c17b629021c34d..866a182baf42967f72f6a0d87866fcf52fdd30a7 100644
--- a/Source/core/page/PageSerializer.h
+++ b/Source/core/page/PageSerializer.h
@@ -73,7 +73,7 @@ private:
 
     // Serializes the stylesheet back to text and adds it to the resources if URL is not-empty.
     // It also adds any resources included in that stylesheet (including any imported stylesheets and their own resources).
-    void serializeCSSStyleSheet(CSSStyleSheet*, const KURL&);
+    void serializeCSSStyleSheet(CSSStyleSheet&, const KURL&);
 
     bool shouldAddURL(const KURL&);