Implement the Notification `timestamp` property

content/public/common/platform_notification_data.h

Index: content/public/common/platform_notification_data.h
diff --git a/content/public/common/platform_notification_data.h b/content/public/common/platform_notification_data.h
index 4a9609ef169c02cdf566acd59cadd54348004cff..65a8891caec3439a4410cc7c220bf1284076d03b 100644
--- a/content/public/common/platform_notification_data.h
+++ b/content/public/common/platform_notification_data.h
@@ -11,6 +11,7 @@
 #include <vector>
 
 #include "base/strings/string16.h"
+#include "base/time/time.h"
 #include "content/common/content_export.h"
 #include "url/gurl.h"
 
@@ -70,6 +71,9 @@ struct CONTENT_EXPORT PlatformNotificationData {
   // Vibration API. https://www.w3.org/TR/vibration/
   std::vector<int> vibration_pattern;
 
+  // The time at which the event the notification represents took place.
+  base::Time timestamp;
+
   // Whether default notification indicators (sound, vibration, light) should
   // be suppressed.
   bool silent = false;