Add extra headers argument so we don't pass headers in the body.

content/browser/download/mhtml_extra_parts_impl.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_MHTML_EXTRA_PARTS_IMPL_H_
 #define CONTENT_BROWSER_DOWNLOAD_MHTML_EXTRA_PARTS_IMPL_H_
 
 #include "content/public/browser/mhtml_extra_parts.h"
 
 namespace content {
 
 // Data fields used to build an additional MHTML part in the output file.
 struct MHTMLExtraDataPart {
   std::string content_type;
   std::string content_location;
+  std::string extra_headers;
   std::string body;
+
+  MHTMLExtraDataPart();
+  ~MHTMLExtraDataPart();
+  MHTMLExtraDataPart(const MHTMLExtraDataPart& other);
 };
 
 // Class used as a data object for WebContents UserData to represent an MHTML
 // part that we plan to write into the output MHTML file.  Each MHTMLExtraPart
 // object in the contained vector lets us hold enough information to generate
 // one MHTML part.  This allows arbitrary extra MHTML parts to be added into the
 // complete file.  For instance, this might be used for gathering load time
 // signals in debug code for analysis.
 class MHTMLExtraPartsImpl : public content::MHTMLExtraParts {
  public:
   MHTMLExtraPartsImpl();
   ~MHTMLExtraPartsImpl() override;
 
   // Return the vector of parts to be serialized.
   const std::vector<MHTMLExtraDataPart>& parts() const { return parts_; }
 
   // Return the number of extra parts added.
   int64_t size() override;
 
   // Creates a MHTMLExtraDataPart and adds it to our vector of parts.
   void AddExtraMHTMLPart(const std::string& content_type,
                          const std::string& content_location,
+                         const std::string& extra_headers,
                          const std::string& body) override;
 
  private:
   std::vector<MHTMLExtraDataPart> parts_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_MHTML_EXTRA_PARTS_IMPL_H_