chromeos: Migrate SignOut and RequestRestartForUpdate to mojo 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 afb8b68adbf0941b46a29c9c1f5d746b494cddd8..fac42de7cd111d6db699415fe1ea8e70e82c6177 100644
--- a/ash/public/interfaces/system_tray.mojom
+++ b/ash/public/interfaces/system_tray.mojom
@@ -72,4 +72,11 @@ interface SystemTrayClient {
 
   // Shows UI for changing proxy settings.
   ShowProxySettings();
+
+  // Attempts to sign out the user.
+  // TODO(jamescook): Migrate to a user or login service. crbug.com/665582
+  SignOut();
+
+  // Attempts to restart the system for update.
+  RequestRestartForUpdate();
 };