Make sure observers are not nullptr

ui/gfx/display_change_notifier_unittest.cc

Index: ui/gfx/display_change_notifier_unittest.cc
diff --git a/ui/gfx/display_change_notifier_unittest.cc b/ui/gfx/display_change_notifier_unittest.cc
index 47b5bd064b64c2c3ff574af9fd751d9dd4459925..9c1dda2fdcae1fe0672b564a1b3e9161f6e6b295 100644
--- a/ui/gfx/display_change_notifier_unittest.cc
+++ b/ui/gfx/display_change_notifier_unittest.cc
@@ -70,13 +70,6 @@ TEST(DisplayChangeNotifierTest, AddObserver_Smoke) {
   EXPECT_EQ(1, observer.display_added());
 }
 
-TEST(DisplayChangeNotifierTest, AddObserver_Null) {
-  DisplayChangeNotifier change_notifier;
-
-  change_notifier.AddObserver(NULL);
-  // Should not crash.
-}
-
 TEST(DisplayChangeNotifier, RemoveObserver_Smoke) {
   DisplayChangeNotifier change_notifier;
   MockDisplayObserver observer;
@@ -93,13 +86,6 @@ TEST(DisplayChangeNotifier, RemoveObserver_Smoke) {
   EXPECT_EQ(0, observer.display_added());
 }
 
-TEST(DisplayChangeNotifierTest, RemoveObserver_Null) {
-  DisplayChangeNotifier change_notifier;
-
-  change_notifier.RemoveObserver(NULL);
-  // Should not crash.
-}
-
 TEST(DisplayChangeNotifierTest, RemoveObserver_Unknown) {
   DisplayChangeNotifier change_notifier;
   MockDisplayObserver observer;