Get signals working in the EXTRA_DATA section of MHTML

content/public/browser/mhtml_extra_parts.h

Index: content/public/browser/mhtml_extra_parts.h
diff --git a/content/public/browser/mhtml_extra_parts.h b/content/public/browser/mhtml_extra_parts.h
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+// 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& body) = 0;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_MHTML_EXTRA_DATA_H_