Fix DialogPlate transparency bug.

See bug for screenshot.

Bug: b:111596959
Change-Id: I96ebd18bcb7d4905df5e539a5ace36154e49c146
Reviewed-on: https://chromium-review.googlesource.com/1142225
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576186}

ash/assistant/ui/dialog_plate/dialog_plate.cc

@@ -103,6 +103,7 @@ void DialogPlate::InitLayout() {
   input_modality_layout_container->SetLayoutManager(
       std::make_unique<views::FillLayout>());
   input_modality_layout_container->SetPaintToLayer();
+  input_modality_layout_container->layer()->SetFillsBoundsOpaquely(false);
   input_modality_layout_container->layer()->SetMasksToBounds(true);
 
   InitKeyboardLayoutContainer(input_modality_layout_container);