chromeos: Introduce SetClient() on ash::mojom::SystemTray interface

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 fac42de7cd111d6db699415fe1ea8e70e82c6177..80516e01513fbd12dd0f241a723a6cd9fd320208 100644
--- a/ash/public/interfaces/system_tray.mojom
+++ b/ash/public/interfaces/system_tray.mojom
@@ -6,6 +6,9 @@ module ash.mojom;
 
 // Allows clients (e.g. Chrome browser) to control the ash system tray menu.
 interface SystemTray {
+  // Sets the client interface.
+  SetClient(SystemTrayClient client);
+
   // 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);