Fix invisible folder name

The folder name becomes invisible because parent view's layer is set not
drawn [1]. This CL sets folder header view's own layer to fix the issue.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1302413

Bug: 902920
Change-Id: Ib32d3c55d8e044693f81e8bfae7547b989c1c80d
Reviewed-on: https://chromium-review.googlesource.com/c/1330762Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607333}

ash/app_list/views/folder_header_view.cc

@@ -80,6 +80,9 @@ FolderHeaderView::FolderHeaderView(FolderHeaderViewDelegate* delegate)
   folder_name_view_->SetTextColor(kFolderNameColor);
   folder_name_view_->set_controller(this);
   AddChildView(folder_name_view_);
+
+  SetPaintToLayer();
+  layer()->SetFillsBoundsOpaquely(false);
 }
 
 FolderHeaderView::~FolderHeaderView() {