mash: Move SystemTrayDelegate ownership to WmShell

ash/system/chromeos/tray_display_unittest.cc

Index: ash/system/chromeos/tray_display_unittest.cc
diff --git a/ash/system/chromeos/tray_display_unittest.cc b/ash/system/chromeos/tray_display_unittest.cc
index 306611d2a27d58e72b1f2669c471fbfab06cb2ae..ce5a878741fb6ea7bcde1baf8bfeffe57466ce08 100644
--- a/ash/system/chromeos/tray_display_unittest.cc
+++ b/ash/system/chromeos/tray_display_unittest.cc
@@ -443,9 +443,7 @@ TEST_F(TrayDisplayTest, UpdateDuringDisplayConfigurationChange) {
 }
 
 TEST_F(TrayDisplayTest, DisplayNotifications) {
-  test::TestSystemTrayDelegate* tray_delegate =
-      static_cast<test::TestSystemTrayDelegate*>(
-          Shell::GetInstance()->system_tray_delegate());
+  test::TestSystemTrayDelegate* tray_delegate = GetSystemTrayDelegate();
   tray_delegate->set_should_show_display_notification(true);
 
   UpdateDisplay("400x400");
@@ -554,9 +552,7 @@ TEST_F(TrayDisplayTest, DisplayNotifications) {
 }
 
 TEST_F(TrayDisplayTest, DisplayConfigurationChangedTwice) {
-  test::TestSystemTrayDelegate* tray_delegate =
-      static_cast<test::TestSystemTrayDelegate*>(
-          Shell::GetInstance()->system_tray_delegate());
+  test::TestSystemTrayDelegate* tray_delegate = GetSystemTrayDelegate();
   tray_delegate->set_should_show_display_notification(true);
 
   UpdateDisplay("400x400,200x200");
@@ -583,9 +579,7 @@ TEST_F(TrayDisplayTest, DisplayConfigurationChangedTwice) {
 TEST_F(TrayDisplayTest, UpdateAfterSuppressDisplayNotification) {
   UpdateDisplay("400x400,200x200");
 
-  test::TestSystemTrayDelegate* tray_delegate =
-      static_cast<test::TestSystemTrayDelegate*>(
-          Shell::GetInstance()->system_tray_delegate());
+  test::TestSystemTrayDelegate* tray_delegate = GetSystemTrayDelegate();
   tray_delegate->set_should_show_display_notification(true);
 
   // rotate the second.