[NTP::Cleanup] Store tab id as int in SnippetArticle.

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>
 
@@ skipping 20 matching lines @@
   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;
+  int tab_id;
   // Underlying offline page identifier.
   int64_t offline_page_id = 0;
 };
 
 // Contains additional data for notification-worthy suggestions.
 struct NotificationExtra {
   // Deadline for showing notification. If the deadline is past, the
   // notification is no longer fresh and no notification should be sent. If the
   // deadline passes while a notification is up, it should be canceled.
   base::Time deadline;
@@ skipping 112 matching lines @@
   std::unique_ptr<NotificationExtra> notification_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_