Get signals working in the EXTRA_DATA section of MHTML

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 body;
+};
+
+// Class used as a data object for WebContents UserData to represent a MHTML

[fgorski, 2017/04/05 18:59:12]

nit: an MHTML

[Pete Williamson, 2017/04/05 22:11:30]

Done.

+// 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;

[fgorski, 2017/04/05 18:59:12]

Could you make this one const as well? Here and in the interface.

[Pete Williamson, 2017/04/05 22:11:30]

Changing this to const produces a build error: 
error: 'const' type qualifier on return type has no effect
[-Werror,-Wignored-qualifiers]

I think it is basically saying there is no point in having a const int as a
return value, since the value is getting copied anyway.  It could be potentially
made into a const reference, is that your suggestion?

Left as is.

[fgorski, 2017/04/05 22:42:38]

Acknowledged. Good point. I blanked on return type.

+
+  // 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& body) override;
+
+ private:
+  std::vector<MHTMLExtraDataPart> parts_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_DOWNLOAD_MHTML_EXTRA_PARTS_IMPL_H_