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..877b64c9fd8373a34d9efe4b8c9735c2db041f57 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 an auxiliary DOM tree, i.e. shadow tree, that needs to be
+    // serialized.
+    virtual std::pair<Node*, Element*> GetAuxiliaryDOMTree(
+        const Element&) const {
+      return std::pair<Node*, Element*>();
+    }
   };
 
   // Constructs a serializer that will write output to the given deque of