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 8a7b3664d701ca8e6d801d7f86382badb77de830..a42e43a74c4d633c5f5408cb8239562b3cc4e396 100644
--- a/third_party/WebKit/Source/core/frame/FrameSerializer.h
+++ b/third_party/WebKit/Source/core/frame/FrameSerializer.h
@@ -108,6 +108,13 @@ class CORE_EXPORT FrameSerializer final {
       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*>();
+    }
+
     virtual bool ShouldCollectProblemMetric() { return false; }
   };