Disable ephemeral apps after they stop running

apps/app_lifetime_monitor.h

Index: apps/app_lifetime_monitor.h
diff --git a/apps/app_lifetime_monitor.h b/apps/app_lifetime_monitor.h
index 9e4aa561f64acddfbb94b6d143a18fcadd81f1cf..bc46e0e992f79db0c9469147f999e8c738992d2d 100644
--- a/apps/app_lifetime_monitor.h
+++ b/apps/app_lifetime_monitor.h
@@ -31,19 +31,18 @@ class AppLifetimeMonitor : public KeyedService,
   class Observer {
    public:
     // Called when the app starts running.
-    virtual void OnAppStart(Profile* profile, const std::string& app_id) = 0;
+    virtual void OnAppStart(Profile* profile, const std::string& app_id) {}
     // Called when the app becomes active to the user, i.e. it opens a window.
-    virtual void OnAppActivated(Profile* profile,
-                                const std::string& app_id) = 0;
+    virtual void OnAppActivated(Profile* profile, const std::string& app_id) {}
     // Called when the app becomes inactive to the user.
-    virtual void OnAppDeactivated(Profile* profile,
-                                  const std::string& app_id) = 0;
+    virtual void OnAppDeactivated(Profile* profile, const std::string& app_id) {
+    }
     // Called when the app stops running.
-    virtual void OnAppStop(Profile* profile, const std::string& app_id) = 0;
+    virtual void OnAppStop(Profile* profile, const std::string& app_id) {}
     // Called when chrome is about to terminate. This gives observers a chance
     // to do something before the apps shut down. This is a system-wide event
     // so there is no associated profile and app id.
-    virtual void OnChromeTerminating() = 0;
+    virtual void OnChromeTerminating() {}
 
    protected:
     virtual ~Observer() {}