[Aura] Refactor and update app list window.

ui/aura_shell/shell.cc

Index: ui/aura_shell/shell.cc
diff --git a/ui/aura_shell/shell.cc b/ui/aura_shell/shell.cc
index b439de1090e7a796cb25e534e0c157144f6378b3..84941c8ca7aa1fcbffc34ad68d8027d2d6ea78f0 100644
--- a/ui/aura_shell/shell.cc
+++ b/ui/aura_shell/shell.cc
@@ -12,6 +12,7 @@
 #include "ui/aura/desktop.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_types.h"
+#include "ui/aura_shell/app_list.h"
 #include "ui/aura_shell/default_container_event_filter.h"
 #include "ui/aura_shell/default_container_layout_manager.h"
 #include "ui/aura_shell/desktop_event_filter.h"
@@ -217,6 +218,12 @@ void Shell::ToggleOverview() {
     workspace_controller_->ToggleOverview();
 }
 
+void Shell::ToggleAppList() {
+  if (!app_list_.get())
+    app_list_.reset(new internal::AppList);
+  app_list_->SetVisible(!app_list_->IsVisible());
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Shell, private: