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 bd9b0494e7a3136d2d772352125b1913cc4a41eb..89362dd8dbb65d03196c6261a212bd9389d94bb9 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);