MacViews: Fix crash when EventMonitorMac receives a NSSystemDefinedEvent

ui/views/event_monitor_mac.mm

Index: ui/views/event_monitor_mac.mm
diff --git a/ui/views/event_monitor_mac.mm b/ui/views/event_monitor_mac.mm
index 1de603634cf09a70a356573cd83abc41ef44e5f5..cb3e54e9a1c7e2550b6c0e1b71d160f2be28200e 100644
--- a/ui/views/event_monitor_mac.mm
+++ b/ui/views/event_monitor_mac.mm
@@ -42,7 +42,8 @@ EventMonitorMac::EventMonitorMac(ui::EventHandler* event_handler,
       handler:^NSEvent*(NSEvent* event) {
           if (!target_window || [event window] == target_window) {
             scoped_ptr<ui::Event> ui_event = ui::EventFromNative(event);
-            event_handler->OnEvent(ui_event.get());
+            if (ui_event)
+              event_handler->OnEvent(ui_event.get());
           }
           return event;
       }];