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

content/public/browser/mhtml_extra_parts.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_PUBLIC_BROWSER_MHTML_EXTRA_PARTS_H_
 #define CONTENT_PUBLIC_BROWSER_MHTML_EXTRA_PARTS_H_
 
 #include "base/supports_user_data.h"
 #include "content/public/browser/web_contents.h"
 
 namespace content {
 
 // Interface class for adding an extra part to MHTML when doing MHTML
 // generation.  To use it, get it from the web contents with FromWebContents,
 // then call AddExtraMHTMLPart to add a new part.
 class CONTENT_EXPORT MHTMLExtraParts : public base::SupportsUserData::Data {
  public:
   // Retrieves the extra data from the web contents.
   static MHTMLExtraParts* FromWebContents(WebContents* contents);
 
   // Add an extra MHTML part to the data structure stored by the WebContents.
   // This will take care of generating the boundary line.  This will also set
   // the content-type and content-location headers to the values provided, and
   // use the body provided.
   virtual void AddExtraMHTMLPart(const std::string& content_type,
                                  const std::string& content_location,
+                                 const std::string& extra_headers,
                                  const std::string& body) = 0;
 
   // Returns the number of extra parts added.
   virtual int64_t size() = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_MHTML_EXTRA_DATA_H_