Update from https://crrev.com/327068

base/observer_list.h

Index: base/observer_list.h
diff --git a/base/observer_list.h b/base/observer_list.h
index f7b92679e9e37af4d3c6d6492da93aed9789cd4e..9ea344deb7aa5e44db447537d6b0243e688944aa 100644
--- a/base/observer_list.h
+++ b/base/observer_list.h
@@ -143,17 +143,18 @@ ObserverListBase<ObserverType>::Iterator::~Iterator() {
 template <class ObserverType>
 ObserverType* ObserverListBase<ObserverType>::Iterator::GetNext() {
   if (!list_.get())
-    return NULL;
+    return nullptr;
   ListType& observers = list_->observers_;
   // Advance if the current element is null
   size_t max_index = std::min(max_index_, observers.size());
   while (index_ < max_index && !observers[index_])
     ++index_;
-  return index_ < max_index ? observers[index_++] : NULL;
+  return index_ < max_index ? observers[index_++] : nullptr;
 }
 
 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!";
@@ -164,11 +165,12 @@ 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()) {
     if (notify_depth_) {
-      *it = 0;
+      *it = nullptr;
     } else {
       observers_.erase(it);
     }
@@ -190,7 +192,7 @@ void ObserverListBase<ObserverType>::Clear() {
   if (notify_depth_) {
     for (typename ListType::iterator it = observers_.begin();
       it != observers_.end(); ++it) {
-      *it = 0;
+      *it = nullptr;
     }
   } else {
     observers_.clear();
@@ -200,8 +202,8 @@ void ObserverListBase<ObserverType>::Clear() {
 template <class ObserverType>
 void ObserverListBase<ObserverType>::Compact() {
   observers_.erase(
-      std::remove(observers_.begin(), observers_.end(),
-                  static_cast<ObserverType*>(NULL)), observers_.end());
+      std::remove(observers_.begin(), observers_.end(), nullptr),
+      observers_.end());
 }
 
 template <class ObserverType, bool check_empty = false>
@@ -233,7 +235,7 @@ class ObserverList : public ObserverListBase<ObserverType> {
       ObserverListBase<ObserverType>::Iterator it_inside_observer_macro( \
           &observer_list);                                               \
       ObserverType* obs;                                                 \
-      while ((obs = it_inside_observer_macro.GetNext()) != NULL)         \
+      while ((obs = it_inside_observer_macro.GetNext()) != nullptr)         \
         obs->func;                                                       \
     }                                                                    \
   } while (0)