Make sure observers are not nullptr

ui/gfx/display_change_notifier_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/display_change_notifier.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/display.h"
 #include "ui/gfx/display_observer.h"
 
@@ skipping 52 matching lines @@
   change_notifier.NotifyDisplaysChanged(
     std::vector<Display>(), std::vector<Display>(1, Display()));
   EXPECT_EQ(0, observer.display_added());
 
   change_notifier.AddObserver(&observer);
   change_notifier.NotifyDisplaysChanged(
     std::vector<Display>(), std::vector<Display>(1, Display()));
   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;
 
   change_notifier.NotifyDisplaysChanged(
     std::vector<Display>(), std::vector<Display>(1, Display()));
   EXPECT_EQ(0, observer.display_added());
 
   change_notifier.AddObserver(&observer);
   change_notifier.RemoveObserver(&observer);
 
   change_notifier.NotifyDisplaysChanged(
     std::vector<Display>(), std::vector<Display>(1, Display()));
   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;
 
   change_notifier.RemoveObserver(&observer);
   // Should not crash.
 }
 
 TEST(DisplayChangeNotifierTest, NotifyDisplaysChanged_Removed) {
   DisplayChangeNotifier change_notifier;
@@ skipping 372 matching lines @@
   change_notifier.NotifyDisplaysChanged(old_displays, new_displays);
   EXPECT_EQ(1, observer.display_changed());
   uint32_t metrics = DisplayObserver::DISPLAY_METRIC_BOUNDS |
                          DisplayObserver::DISPLAY_METRIC_ROTATION |
                          DisplayObserver::DISPLAY_METRIC_WORK_AREA |
                          DisplayObserver::DISPLAY_METRIC_DEVICE_SCALE_FACTOR;
   EXPECT_EQ(metrics, observer.latest_metrics_change());
 }
 
 } // namespace gfx