Support serializing shadow DOM to MHTML

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..83a0a6f5115add18f351a9b1beb086e7e0a4dd64 100644
--- a/third_party/WebKit/Source/core/frame/FrameSerializer.h
+++ b/third_party/WebKit/Source/core/frame/FrameSerializer.h
@@ -107,6 +107,13 @@ class CORE_EXPORT FrameSerializer final {
     virtual Vector<Attribute> GetCustomAttributes(const Element&) {
       return Vector<Attribute>();
     }
+
+    // Returns the root of auxiliary DOM tree, i.e. shadow tree, if it needs to
+    // be serialized. The additional element to enclose the serialized
+    // auxiliary DOM tree may be returned.
+    virtual Node* GetAuxiliaryDOMTree(const Element&, Element**) const {
+      return nullptr;
+    }
   };
 
   // Constructs a serializer that will write output to the given deque of