Implement support for silent notifications.

ui/message_center/notification.cc

Index: ui/message_center/notification.cc
diff --git a/ui/message_center/notification.cc b/ui/message_center/notification.cc
index 48c84894b3239ae992f601a72e922f91d0dd3af9..605662083dd0c0ec3d81bb69eda73b5bbf92eb89 100644
--- a/ui/message_center/notification.cc
+++ b/ui/message_center/notification.cc
@@ -30,7 +30,8 @@ RichNotificationData::RichNotificationData()
       timestamp(base::Time::Now()),
       progress(0),
       should_make_spoken_feedback_for_popup_updates(true),
-      clickable(true) {}
+      clickable(true),
+      silent(false) {}
 
 RichNotificationData::RichNotificationData(const RichNotificationData& other)
     : priority(other.priority),
@@ -44,7 +45,8 @@ RichNotificationData::RichNotificationData(const RichNotificationData& other)
       buttons(other.buttons),
       should_make_spoken_feedback_for_popup_updates(
           other.should_make_spoken_feedback_for_popup_updates),
-      clickable(other.clickable) {}
+      clickable(other.clickable),
+      silent(other.silent) {}
 
 RichNotificationData::~RichNotificationData() {}