[remote suggestions] Attach the fetch time to RemoteSnippets, ContentSnippets and SnippetArticle

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 b4e64f887dfb53e56081c18f7a463d9f31d4a724..ea17274c45fe2402060c6c2e6830f3b42cbf9390 100644
--- a/components/ntp_snippets/content_suggestion.h
+++ b/components/ntp_snippets/content_suggestion.h
@@ -151,6 +151,11 @@ class ContentSuggestion {
   void set_notification_extra(
       std::unique_ptr<NotificationExtra> notification_extra);
 
+  const base::Time& fetch_date() const { return fetch_date_; }
+  void set_fetch_date(const base::Time& fetch_date) {
+    fetch_date_ = fetch_date;
+  }
+
  private:
   ID id_;
   GURL url_;
@@ -163,6 +168,11 @@ class ContentSuggestion {
   std::unique_ptr<RecentTabSuggestionExtra> recent_tab_suggestion_extra_;
   std::unique_ptr<NotificationExtra> notification_extra_;
 
+  // The time when the remote suggestion was fetched from the server. This field
+  // is only populated when the ContentSuggestion is created from a
+  // RemoteSuggestion.
+  base::Time fetch_date_;
+
   DISALLOW_COPY_AND_ASSIGN(ContentSuggestion);
 };