[Offline Pages] Adding missing image/CSS detection in FrameSerializer.

third_party/WebKit/Source/core/frame/FrameSerializer.h

Index: third_party/WebKit/Source/core/frame/FrameSerializer.h
diff --git a/third_party/WebKit/Source/core/frame/FrameSerializer.h b/third_party/WebKit/Source/core/frame/FrameSerializer.h
index b53d02d984a0f7defee3763e8f5217ef5ff9e5fd..b20268b5a90cadb89eb56858720cb96376826221 100644
--- a/third_party/WebKit/Source/core/frame/FrameSerializer.h
+++ b/third_party/WebKit/Source/core/frame/FrameSerializer.h
@@ -51,6 +51,7 @@ class Element;
 class FontResource;
 class ImageResourceContent;
 class LocalFrame;
+class Node;
 class SharedBuffer;
 class StylePropertySet;
 
@@ -107,6 +108,9 @@ class CORE_EXPORT FrameSerializer final {
     virtual Vector<Attribute> GetCustomAttributes(const Element&) {
       return Vector<Attribute>();
     }
+
+    // Visits a node during serialization.
+    virtual void VisitNode(const Node*) {}
   };
 
   // Constructs a serializer that will write output to the given deque of