Adds TraySessionLengthLimitTest to ash_unittests.

ash/test/ash_test_base.cc

Index: ash/test/ash_test_base.cc
diff --git a/ash/test/ash_test_base.cc b/ash/test/ash_test_base.cc
index 335972d40be871c084728ce7a99fb24b10662115..30ca1be45ad08e01873511d3c4deb6dfd7d9505c 100644
--- a/ash/test/ash_test_base.cc
+++ b/ash/test/ash_test_base.cc
@@ -16,6 +16,7 @@
 #include "ash/test/display_manager_test_api.h"
 #include "ash/test/test_session_state_delegate.h"
 #include "ash/test/test_shell_delegate.h"
+#include "ash/test/test_system_tray_delegate.h"
 #include "ash/wm/coordinate_conversion.h"
 #include "ash/wm/window_positioner.h"
 #include "base/command_line.h"
@@ -286,6 +287,11 @@ TestScreenshotDelegate* AshTestBase::GetScreenshotDelegate() {
   return ash_test_helper_->test_screenshot_delegate();
 }
 
+TestSystemTrayDelegate* AshTestBase::GetSystemTrayDelegate() {
+  return static_cast<TestSystemTrayDelegate*>(
+      Shell::GetInstance()->system_tray_delegate());
+}
+
 void AshTestBase::SetSessionStarted(bool session_started) {
   ash_test_helper_->test_shell_delegate()->test_session_state_delegate()->
       SetActiveUserSessionStarted(session_started);