ash: Use system tray mojo interface to show system update tray icon

ash/public/interfaces/system_tray.mojom

Index: ash/public/interfaces/system_tray.mojom
diff --git a/ash/public/interfaces/system_tray.mojom b/ash/public/interfaces/system_tray.mojom
index 89362dd8dbb65d03196c6261a212bd9389d94bb9..7013d624fc09ddd191ff543b1fac028cabd0580c 100644
--- a/ash/public/interfaces/system_tray.mojom
+++ b/ash/public/interfaces/system_tray.mojom
@@ -4,6 +4,8 @@
 
 module ash.mojom;
 
+import "ash/public/interfaces/update.mojom";
+
 // Allows clients (e.g. Chrome browser) to control the ash system tray menu.
 interface SystemTray {
   // Sets the client interface.
@@ -12,6 +14,11 @@ interface SystemTray {
   // Sets the clock to use 24 hour time formatting if |use_24_hour| is true.
   // Otherwise sets 12 hour time formatting.
   SetUse24HourClock(bool use_24_hour);
+
+  // Shows an icon in the system tray indicating that a software update is
+  // available. Once shown the icon persists until reboot. |severity| and
+  // |factory_reset_required| are used to set the icon, color, and tooltip.
+  ShowUpdateIcon(UpdateSeverity severity, bool factory_reset_required);
 };
 
 // Allows ash system tray to control a client (e.g. Chrome browser). Requests