Fix ChromeVox focus bug in app list folder

Changes:
Change the role of app list folder to GENERIC_CONTAINER from BUTTON.

BUG=776238

Change-Id: I3c0395678478c873e7328d1cb26bfe1d78043286
Reviewed-on: https://chromium-review.googlesource.com/727562Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510128}

ui/app_list/views/app_list_folder_view.cc

@@ -347,7 +347,7 @@ void AppListFolderView::SetRootLevelDragViewVisible(bool visible) {
 }
 
 void AppListFolderView::GetAccessibleNodeData(ui::AXNodeData* node_data) {
-  node_data->role = ui::AX_ROLE_BUTTON;
+  node_data->role = ui::AX_ROLE_GENERIC_CONTAINER;
   node_data->SetName(accessible_name_);
 }