Implement app list folder management page UI.

ui/app_list/views/app_list_main_view.cc

Index: ui/app_list/views/app_list_main_view.cc
diff --git a/ui/app_list/views/app_list_main_view.cc b/ui/app_list/views/app_list_main_view.cc
index 792c5bd8355bdfa59d059de8d239026b526b92cb..0a83325781007d8ec2d2de7a00eb774120487904 100644
--- a/ui/app_list/views/app_list_main_view.cc
+++ b/ui/app_list/views/app_list_main_view.cc
@@ -12,6 +12,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/strings/string_util.h"
 #include "ui/app_list/app_list_constants.h"
+#include "ui/app_list/app_list_folder_item.h"
 #include "ui/app_list/app_list_item_model.h"
 #include "ui/app_list/app_list_model.h"
 #include "ui/app_list/app_list_view_delegate.h"
@@ -190,7 +191,11 @@ void AppListMainView::OnItemIconLoaded(IconLoader* loader) {
 }
 
 void AppListMainView::ActivateApp(AppListItemModel* item, int event_flags) {
-  item->Activate(event_flags);
+  // TODO(jennyz): Activate the folder via AppListModel notification.
+  if (item->GetAppType() == AppListFolderItem::kAppType)
+    contents_view_->ShowFolderContent(static_cast<AppListFolderItem*>(item));
+  else
+    item->Activate(event_flags);
 }
 
 void AppListMainView::GetShortcutPathForApp(