[NTP] Propagate OfflineId from C++ for Downloads.

components/ntp_snippets/content_suggestion.h

 // Copyright 2016 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 COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTION_H_
 #define COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTION_H_
 
 #include <memory>
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/strings/string16.h"
 #include "base/time/time.h"
 #include "components/ntp_snippets/category.h"
 #include "url/gurl.h"
 
 namespace ntp_snippets {
 
 // DownloadSuggestionExtra contains additional data which is only available for
 // download suggestions.
 struct DownloadSuggestionExtra {
   // The file path of the downloaded file once download completes.
   base::FilePath target_file_path;
   // The effective MIME type of downloaded content.
   std::string mime_type;
+  // Underlying offline page identifier.
+  std::string offline_page_id;
   // Whether or not the download suggestion is a downloaded asset.
   bool is_download_asset = false;

[Marc Treib, 2016/11/16 15:22:58]

Hm, this is now a bit messy... if is_download_asset is true, only
offline_page_id will be valid; otherwise only the other two members will be
valid. Right?
Since I don't really have a better idea: Can we at least add a comment that
states this?
(I guess the alternative would be to have static factory methods
ForAssetDownload and ForOfflinePage that take exactly the required inputs. Not
sure if that's worth it though.)

[vitaliii, 2016/11/16 15:45:08]

It is other way around. When is_download_asset is false, offline_page_id is
valid, otherwise remaining two variables are valid. I added a comment.

 };
 
 // Contains additional data which is only available for recent tab suggestions.
 struct RecentTabSuggestionExtra {
   // Corresponding tab identifier.
   std::string tab_id;
   // Underlying offline page identifier.
   std::string offline_page_id;
 };
 
@@ skipping 105 matching lines @@
   std::unique_ptr<RecentTabSuggestionExtra> recent_tab_suggestion_extra_;
 
   DISALLOW_COPY_AND_ASSIGN(ContentSuggestion);
 };
 
 std::ostream& operator<<(std::ostream& os, const ContentSuggestion::ID& id);
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CONTENT_SUGGESTION_H_