chromeos: Converts more tests to AshTestBase

ash/system/brightness/tray_brightness_unittest.cc

Index: ash/system/brightness/tray_brightness_unittest.cc
diff --git a/ash/system/brightness/tray_brightness_unittest.cc b/ash/system/brightness/tray_brightness_unittest.cc
index 4819859986d8943ac5647efa9de1fe767a8e38ca..e1f4e086285dc9396e958d6601b096b4d40fcadb 100644
--- a/ash/system/brightness/tray_brightness_unittest.cc
+++ b/ash/system/brightness/tray_brightness_unittest.cc
@@ -9,13 +9,13 @@
 #include "ash/shell.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/system_tray_item.h"
-#include "ash/test/ash_test.h"
+#include "ash/test/ash_test_base.h"
 #include "ash/wm/maximize_mode/maximize_mode_controller.h"
 #include "ui/views/view.h"
 
 namespace ash {
 
-class TrayBrightnessTest : public AshTest {
+class TrayBrightnessTest : public test::AshTestBase {
  public:
   TrayBrightnessTest() {}
   ~TrayBrightnessTest() override {}