[Offline Pages] Set up the initial prefetching service.

components/ntp_snippets/content_suggestions_service.h

Index: components/ntp_snippets/content_suggestions_service.h
diff --git a/components/ntp_snippets/content_suggestions_service.h b/components/ntp_snippets/content_suggestions_service.h
index 40d665c78e818a179cf0381e6c4d0cac7046e4c3..4b0e16f3652b9388310af41bf3d7969d596fbf67 100644
--- a/components/ntp_snippets/content_suggestions_service.h
+++ b/components/ntp_snippets/content_suggestions_service.h
@@ -15,6 +15,7 @@
 #include "base/observer_list.h"
 #include "base/optional.h"
 #include "base/scoped_observer.h"
+#include "base/supports_user_data.h"
 #include "base/task/cancelable_task_tracker.h"
 #include "base/time/time.h"
 #include "components/history/core/browser/history_service.h"
@@ -47,6 +48,7 @@ class RemoteSuggestionsProvider;
 // Retrieves suggestions from a number of ContentSuggestionsProviders and serves
 // them grouped into categories. There can be at most one provider per category.
 class ContentSuggestionsService : public KeyedService,
+                                  public base::SupportsUserData,
                                   public ContentSuggestionsProvider::Observer,
                                   public SigninManagerBase::Observer,
                                   public history::HistoryServiceObserver {