Make sure observers are not nullptr

ui/views/widget/widget.cc

Index: ui/views/widget/widget.cc
diff --git a/ui/views/widget/widget.cc b/ui/views/widget/widget.cc
index e78a45ef5bfe3415688dec93b46aa3c2189fffa8..7c7469391627ca7ef0bd33d7d16bda3fab399be6 100644
--- a/ui/views/widget/widget.cc
+++ b/ui/views/widget/widget.cc
@@ -390,6 +390,8 @@ 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);
 }