Implement support for silent notifications.

ui/message_center/notification.h

Index: ui/message_center/notification.h
diff --git a/ui/message_center/notification.h b/ui/message_center/notification.h
index 4f913d79c6ed6560511f1e0ce6853d620c0ddfc9..060cddaef49dcc1156658e010abe6caa60683c0c 100644
--- a/ui/message_center/notification.h
+++ b/ui/message_center/notification.h
@@ -50,6 +50,7 @@ class MESSAGE_CENTER_EXPORT RichNotificationData {
   std::vector<ButtonInfo> buttons;
   bool should_make_spoken_feedback_for_popup_updates;
   bool clickable;
+  bool silent;
 };
 
 class MESSAGE_CENTER_EXPORT Notification {
@@ -103,6 +104,10 @@ class MESSAGE_CENTER_EXPORT Notification {
   int priority() const { return optional_fields_.priority; }
   void set_priority(int priority) { optional_fields_.priority = priority; }
 
+  // This property currently has no effect on non-Android platforms.
+  bool silent() const { return optional_fields_.silent; }
+  void set_silent(bool silent) { optional_fields_.silent = silent; }
+
   base::Time timestamp() const { return optional_fields_.timestamp; }
   void set_timestamp(const base::Time& timestamp) {
     optional_fields_.timestamp = timestamp;