Fix use after free for cached_displays_.

ui/display/manager/chromeos/test/test_native_display_delegate.h

Index: ui/display/manager/chromeos/test/test_native_display_delegate.h
diff --git a/ui/display/manager/chromeos/test/test_native_display_delegate.h b/ui/display/manager/chromeos/test/test_native_display_delegate.h
index 1b875f6ba40093fec7caee55802c5ed28fca79c4..c994d73ae39b739fa213c14f140f2e92486583d1 100644
--- a/ui/display/manager/chromeos/test/test_native_display_delegate.h
+++ b/ui/display/manager/chromeos/test/test_native_display_delegate.h
@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "ui/display/manager/chromeos/test/action_logger.h"
 #include "ui/display/manager/chromeos/test/action_logger_util.h"
 #include "ui/display/types/native_display_delegate.h"
@@ -18,6 +19,7 @@ namespace ui {
 
 class ActionLogger;
 class DisplaySnapshot;
+class NativeDisplayObserver;
 
 namespace test {
 
@@ -111,6 +113,8 @@ class TestNativeDisplayDelegate : public NativeDisplayDelegate {
 
   ActionLogger* log_;  // Not owned.
 
+  base::ObserverList<NativeDisplayObserver> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(TestNativeDisplayDelegate);
 };