Introduces --app flag to athena to load an app.

apps/shell/browser/default_shell_browser_main_delegate.cc

Index: apps/shell/browser/default_shell_browser_main_delegate.cc
diff --git a/apps/shell/browser/default_shell_browser_main_delegate.cc b/apps/shell/browser/default_shell_browser_main_delegate.cc
index 3efabbb36d7f5a88dba923e70a11f74d618a53c3..b9a8baffe1e33fd2d5680bff211fa5b55b38af24 100644
--- a/apps/shell/browser/default_shell_browser_main_delegate.cc
+++ b/apps/shell/browser/default_shell_browser_main_delegate.cc
@@ -29,7 +29,9 @@ void DefaultShellBrowserMainDelegate::Start(
     extensions::ShellExtensionSystem* extension_system =
         static_cast<extensions::ShellExtensionSystem*>(
             extensions::ExtensionSystem::Get(browser_context));
-    extension_system->LoadAndLaunchApp(app_absolute_dir);
+    if (!extension_system->LoadApp(app_absolute_dir))
+      return;
+    extension_system->LaunchApp();
   } else {
     LOG(ERROR) << "--" << kAppSwitch << " unset; boredom is in your future";
   }