Separate the opacity of AppsContainerView and PageSwitcher

My CL r681543 removed the layer for PageSwitcher and reused
a new AppsContainerView's layer, but it turned out this was wrong
since the opacity of parent layer affects the descendent layer
opacities.

This CL revives the PageSwitcher layer to recover the original
behavior of the opacity when dragging the app-list. Since all of
AppsContainerView's children have their own layers, this is okay
to be ui::LAYER_NOT_DRAWN and hence it won't affect the drawing
performance.

Bug: 988494
Test: manually
Change-Id: If1913b5fa426b7ee63bb25ac7fef180375cdd30d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722408Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681903}

ash/app_list/views/apps_container_view.cc

@@ -61,8 +61,7 @@ constexpr float kSuggestionChipOpacityEndProgress = 1;
 AppsContainerView::AppsContainerView(ContentsView* contents_view,
                                      AppListModel* model)
     : contents_view_(contents_view) {
-  SetPaintToLayer();
-  layer()->SetFillsBoundsOpaquely(false);
+  SetPaintToLayer(ui::LAYER_NOT_DRAWN);
 
   suggestion_chip_container_view_ =
       new SuggestionChipContainerView(contents_view);
@@ -198,7 +197,7 @@ void AppsContainerView::UpdateYPositionAndOpacity() {
               (AppListConfig::instance().all_apps_opacity_end_px() - start_px),
           0.f),
       1.0f);
-  layer()->SetOpacity(should_restore_opacity ? 1.0f : opacity);
+  page_switcher_->layer()->SetOpacity(should_restore_opacity ? 1.0f : opacity);
 
   const float progress =
       contents_view_->app_list_view()->GetAppListTransitionProgress();

ash/app_list/views/page_switcher.cc

@@ -190,6 +190,8 @@ PageSwitcher::PageSwitcher(ash::PaginationModel* model,
       buttons_(new views::View),
       vertical_(vertical),
       is_tablet_mode_(is_tablet_mode) {
+  SetPaintToLayer();
+  layer()->SetFillsBoundsOpaquely(false);
   if (vertical_) {
     buttons_->SetLayoutManager(std::make_unique<views::BoxLayout>(
         views::BoxLayout::Orientation::kVertical, gfx::Insets(),