[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 {
+  DownloadSuggestionExtra();
+  ~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.
+  int64_t offline_page_id = 0;
   // 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;
 };
 
 // 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;
+  int64_t offline_page_id;
 };
 
 // A content suggestion for the new tab page, which can be an article or an
 // offline page, for example.
 class ContentSuggestion {
  public:
   class ID {
    public:
     ID(Category category, const std::string& id_within_category)
         : category_(category), id_within_category_(id_within_category) {}
@@ skipping 97 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_