Address a race condition in displaying notifications

content/browser/notifications/platform_notification_context_impl.h

Index: content/browser/notifications/platform_notification_context_impl.h
diff --git a/content/browser/notifications/platform_notification_context_impl.h b/content/browser/notifications/platform_notification_context_impl.h
index 67405dc0de6d0ac82e449dce2ff6cdd5c10399d4..97709f47062b17b8d1c217ff01ee496b609cdc6e 100644
--- a/content/browser/notifications/platform_notification_context_impl.h
+++ b/content/browser/notifications/platform_notification_context_impl.h
@@ -9,6 +9,7 @@
 #include <memory>
 #include <set>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "base/callback.h"
@@ -16,6 +17,7 @@
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/time/time.h"
 #include "content/browser/notifications/notification_id_generator.h"
 #include "content/browser/service_worker/service_worker_context_observer.h"
 #include "content/common/content_export.h"
@@ -26,6 +28,7 @@
 class GURL;
 
 namespace base {
+class Clock;
 class SequencedTaskRunner;
 }
 
@@ -207,6 +210,12 @@ class CONTENT_EXPORT PlatformNotificationContextImpl
 
   NotificationIdGenerator notification_id_generator_;
 
+  std::unique_ptr<base::Clock> clock_;
+
+  // Stores the recently displayed notification ids together with the time at
+  // which they were displayed.
+  std::map<std::string, base::Time> recent_notifications_;
+
   // Indicates whether the database should be pruned when it's opened.
   bool prune_database_on_open_ = false;