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

content/browser/download/mhtml_extra_parts_impl.h

Index: content/browser/download/mhtml_extra_parts_impl.h
diff --git a/content/browser/download/mhtml_extra_parts_impl.h b/content/browser/download/mhtml_extra_parts_impl.h
index d1879dc959abc32285ab3344f06d0973b451a79e..ef890ddfad84ff0006de5ccff4f0de9838a47b8c 100644
--- a/content/browser/download/mhtml_extra_parts_impl.h
+++ b/content/browser/download/mhtml_extra_parts_impl.h
@@ -13,7 +13,12 @@ namespace content {
 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
@@ -36,6 +41,7 @@ class MHTMLExtraPartsImpl : public content::MHTMLExtraParts {
   // 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: