Make sure observers are not nullptr

I believe there is no good reason to allow nullptr observer,
and the bug seems to be hitting this. This is to catch such bug early.

BUG=471649

Review URL: https://codereview.chromium.org/1059383003

Cr-Commit-Position: refs/heads/master@{#324299}

base/observer_list.h

@@ -154,6 +154,7 @@ ObserverType* ObserverListBase<ObserverType>::Iterator::GetNext() {
 
 template <class ObserverType>
 void ObserverListBase<ObserverType>::AddObserver(ObserverType* obs) {
+  DCHECK(obs);
   if (std::find(observers_.begin(), observers_.end(), obs)
       != observers_.end()) {
     NOTREACHED() << "Observers can only be added once!";
@@ -164,6 +165,7 @@ void ObserverListBase<ObserverType>::AddObserver(ObserverType* obs) {
 
 template <class ObserverType>
 void ObserverListBase<ObserverType>::RemoveObserver(ObserverType* obs) {
+  DCHECK(obs);
   typename ListType::iterator it =
     std::find(observers_.begin(), observers_.end(), obs);
   if (it != observers_.end()) {

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;

ui/views/widget/widget.cc

@@ -389,6 +389,8 @@ gfx::NativeWindow Widget::GetNativeWindow() const {
 }
 
 void Widget::AddObserver(WidgetObserver* observer) {
+  // Make sure that there is no nullptr in observer list. crbug.com/471649.
+  CHECK(observer);
   observers_.AddObserver(observer);
 }