SuggestionsService: implement SyncServiceObserver

chrome/browser/search/instant_service.h

Index: chrome/browser/search/instant_service.h
diff --git a/chrome/browser/search/instant_service.h b/chrome/browser/search/instant_service.h
index 49f37d3b413ad86fa4db7222e0260287a5dbb016..dccf4e7cc58f2642d3c3021b7c9d3b2b0b0ee461 100644
--- a/chrome/browser/search/instant_service.h
+++ b/chrome/browser/search/instant_service.h
@@ -10,6 +10,7 @@
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "components/history/core/browser/history_types.h"
@@ -184,6 +185,11 @@ class InstantService : public KeyedService,
   // Suggestions Service to fetch server suggestions.
   suggestions::SuggestionsService* suggestions_service_;
 
+  // Subscription to the SuggestionsService.
+  scoped_ptr<
+      suggestions::SuggestionsService::ResponseCallbackList::Subscription>
+      suggestions_subscription_;
+
   // Used for Top Sites async retrieval.
   base::WeakPtrFactory<InstantService> weak_ptr_factory_;