[NTP Snippets] Clear the snippets when the user signs out

components/ntp_snippets/ntp_snippets_service.h

 // Copyright 2015 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_NTP_SNIPPETS_SERVICE_H_
 #define COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 180 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsService);
 };
 
 class NTPSnippetsServiceObserver {
  public:
   // Sent every time the service loads a new set of data.
   virtual void NTPSnippetsServiceLoaded() = 0;
   // Sent when the service is shutting down.
   virtual void NTPSnippetsServiceShutdown() = 0;
+  // Sent every time the service has its data sources cleared.
+  // Note: we use it as signal that the user signed out or disabled sync, but it
+  // can be triggered in other cases, when the suggestions service returns no
+  // hosts. (e.g. the user manually removed all MostLikely tiles.)
+  virtual void NTPSnippetsServiceCleared() = 0;
 
  protected:
   virtual ~NTPSnippetsServiceObserver() {}
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_SERVICE_H_