Fixes tray_session_length_limit_unittest timeout on valgrind.

ash/system/chromeos/session/tray_session_length_limit_unittest.cc

Index: ash/system/chromeos/session/tray_session_length_limit_unittest.cc
diff --git a/ash/system/chromeos/session/tray_session_length_limit_unittest.cc b/ash/system/chromeos/session/tray_session_length_limit_unittest.cc
index e1152ce6f3622ad8a12d8478381b8e398e67ceb5..da203a5455fd0093aa635249e405349a15834438 100644
--- a/ash/system/chromeos/session/tray_session_length_limit_unittest.cc
+++ b/ash/system/chromeos/session/tray_session_length_limit_unittest.cc
@@ -30,7 +30,10 @@ class TraySessionLengthLimitTest : public AshTestBase {
     system_tray->AddTrayItem(tray_session_length_limit_);
   }
 
-  void TearDown() override { AshTestBase::TearDown(); }
+  void TearDown() override {
+    ClearSessionLengthLimit();
+    AshTestBase::TearDown();
+  }
 
  protected:
   void UpdateSessionLengthLimitInMin(int mins) {
@@ -47,7 +50,7 @@ class TraySessionLengthLimitTest : public AshTestBase {
       if ((*iter)->id() == TraySessionLengthLimit::kNotificationId)
         return *iter;
     }
-    return NULL;
+    return nullptr;
   }
 
   void ClearSessionLengthLimit() {
@@ -120,8 +123,6 @@ TEST_F(TraySessionLengthLimitTest, Notification) {
 }
 
 TEST_F(TraySessionLengthLimitTest, RemoveNotification) {
-  message_center::Notification* notification;
-
   // Limit is 15 min.
   UpdateSessionLengthLimitInMin(15);
   EXPECT_TRUE(GetNotification());
@@ -137,7 +138,7 @@ TEST_F(TraySessionLengthLimitTest, RemoveNotification) {
   // Limit is 3 min. The notification should re-appear and should be re-read
   // because of state change.
   UpdateSessionLengthLimitInMin(3);
-  notification = GetNotification();
+  message_center::Notification* notification = GetNotification();
   EXPECT_TRUE(notification);
   EXPECT_TRUE(notification->rich_notification_data().
               should_make_spoken_feedback_for_popup_updates);