Make sure observers are not nullptr

base/observer_list.h

Index: base/observer_list.h
diff --git a/base/observer_list.h b/base/observer_list.h
index f7b92679e9e37af4d3c6d6492da93aed9789cd4e..e473b3250ae58dedc74c1801a500bafe27f377b9 100644
--- a/base/observer_list.h
+++ b/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()) {