[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..8a3e1597159661ba79ae6c0d00769045e0c3989a 100644
--- a/components/ntp_snippets/content_suggestion.h
+++ b/components/ntp_snippets/content_suggestion.h
@@ -24,7 +24,11 @@ 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;

[Bernhard Bauer, 2016/11/16 17:04:02]

Same here -- why is this a string and not an int64_t?

[vitaliii, 2016/11/16 17:28:10]

Done.

   // Whether or not the download suggestion is a downloaded asset.
+  // When this is true, |offline_page_id| is ignored, otherwise
+  // |target_file_path| and |mime_type| are ignored.
   bool is_download_asset = false;
 };