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

components/ntp_snippets/content_suggestion.h

Index: components/ntp_snippets/content_suggestion.h
diff --git a/components/ntp_snippets/content_suggestion.h b/components/ntp_snippets/content_suggestion.h
index 630b7e5ad65efe1740c13d78cd0807a3dea7a7d5..e07738a9a58c2215b33902709d247f75d527d3c8 100644
--- a/components/ntp_snippets/content_suggestion.h
+++ b/components/ntp_snippets/content_suggestion.h
@@ -24,6 +24,8 @@ struct DownloadSuggestionExtra {
   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.

 };