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 15b810a90c1804c1f9adb1250b78e4f1d836ba57..8b391b63e17ad06597009ecd9fc64c65a6890b3a 100644
--- a/ui/views/widget/widget.cc
+++ b/ui/views/widget/widget.cc
@@ -389,6 +389,8 @@ gfx::NativeWindow Widget::GetNativeWindow() const {
 }
 
 void Widget::AddObserver(WidgetObserver* observer) {
+  // TODO(oshima): Remove this CHECK once crbug.com/471649 is resolved.

[sadrul, 2015/04/08 19:12:33]

I actually like this check. Maybe just always keep it and remove the TODO?

[oshima, 2015/04/08 19:17:10]

Sure. I just updated the comment to give more context.

[danakj, 2015/04/08 20:03:45]

Why CHECK instead of DCHECK? It adds code/strings to the binary we ship.

+  CHECK(observer);
   observers_.AddObserver(observer);
 }