[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 9bac241d728ed05d3e3b0188e5c449b55c52ed73..22873fca117dc4a38223f03a2d2a60ba862081a1 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,

[jianli, 2017/04/17 21:52:58]

Why is this needed?

[dewittj, 2017/04/17 22:22:18]

This is needed so that ContentSuggestionsService can own the
SuggestionsObserver.  Other implementations might require
ContentSuggestionsService to actually know about the SuggestionsObserver, or for
the Observer to manage its own lifetime using DeleteSoon.  I had that type of
API before and it did not seem very reliable.

                                   public ContentSuggestionsProvider::Observer,
                                   public SigninManagerBase::Observer,
                                   public history::HistoryServiceObserver {