mash: Port TrayUpdateTest to AshTest

ash/common/test/ash_test.cc

Index: ash/common/test/ash_test.cc
diff --git a/ash/common/test/ash_test.cc b/ash/common/test/ash_test.cc
index aefe98345470a0fad7f3f5e262fc2d538dbf8215..32744b9c82da250a983050b173c90ad0847995bc 100644
--- a/ash/common/test/ash_test.cc
+++ b/ash/common/test/ash_test.cc
@@ -4,6 +4,8 @@
 
 #include "ash/common/test/ash_test.h"
 
+#include "ash/common/shelf/wm_shelf.h"
+#include "ash/common/system/status_area_widget.h"
 #include "ash/common/test/ash_test_impl.h"
 #include "ash/common/test/test_session_state_delegate.h"
 #include "ash/common/wm_root_window_controller.h"
@@ -33,6 +35,11 @@ WmShelf* AshTest::GetPrimaryShelf() {
       ->GetShelf();
 }
 
+// static
+SystemTray* AshTest::GetPrimarySystemTray() {
+  return GetPrimaryShelf()->GetStatusAreaWidget()->system_tray();
+}
+
 bool AshTest::SupportsMultipleDisplays() const {
   return test_impl_->SupportsMultipleDisplays();
 }