Use NOT_DRAWN layer for the layer that doesn't have content

This reduces the repainting during hotseat animation.

Bug: 1049355

Change-Id: I36cbdb73f7417494b224c0d71f2efb1a6929c5b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088972Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748534}

ash/shelf/scrollable_shelf_view.cc

@@ -278,6 +278,8 @@ class ScrollableShelfView::ScrollableShelfArrowView
                         shelf_button_delegate) {
     SetInkDropMode(InkDropMode::OFF);
     SetEventTargeter(std::make_unique<views::ViewTargeter>(this));
+    SetPaintToLayer();
+    layer()->SetFillsBoundsOpaquely(false);
   }
   ~ScrollableShelfArrowView() override = default;
 
@@ -500,7 +502,7 @@ void ScrollableShelfView::Init() {
   // needed. Otherwise, the child view without its own layer will be painted on
   // RootView which is beneath |translucent_background_| in ShelfWidget.
   // As a result, the child view will not show.
-  SetPaintToLayer();
+  SetPaintToLayer(ui::LAYER_NOT_DRAWN);
   layer()->SetFillsBoundsOpaquely(false);
 
   // Initialize the shelf container view.
@@ -1824,7 +1826,6 @@ void ScrollableShelfView::PaintGradientZone(const FadeZone& start_rect,
                                             const FadeZone& end_rect) {
   gradient_layer_delegate_->set_start_fade_zone(start_rect);
   gradient_layer_delegate_->set_end_fade_zone(end_rect);
-
   SchedulePaint();
 }
 

ash/shelf/shelf_app_button.cc

@@ -319,6 +319,9 @@ ShelfAppButton::ShelfAppButton(ShelfView* shelf_view,
   // Do not make this interactive, so that events are sent to ShelfView.
   icon_view_->set_can_process_events_within_subtree(false);
 
+  indicator_->SetPaintToLayer();
+  indicator_->layer()->SetFillsBoundsOpaquely(false);
+
   AddChildView(indicator_);
   AddChildView(icon_view_);
   if (is_notification_indicator_enabled_) {

ash/shelf/shelf_container_view.cc

@@ -14,11 +14,11 @@ ShelfContainerView::ShelfContainerView(ShelfView* shelf_view)
 ShelfContainerView::~ShelfContainerView() = default;
 
 void ShelfContainerView::Initialize() {
-  SetPaintToLayer();
+  SetPaintToLayer(ui::LAYER_NOT_DRAWN);
   layer()->SetFillsBoundsOpaquely(false);
   layer()->SetMasksToBounds(true);
 
-  shelf_view_->SetPaintToLayer();
+  shelf_view_->SetPaintToLayer(ui::LAYER_NOT_DRAWN);
   shelf_view_->layer()->SetFillsBoundsOpaquely(false);
   AddChildView(shelf_view_);
 }

ash/shelf/shelf_view.cc

@@ -898,7 +898,7 @@ bool ShelfView::ShouldShowTooltipForChildView(
 
 // static
 void ShelfView::ConfigureChildView(views::View* view) {
-  view->SetPaintToLayer();
+  view->SetPaintToLayer(ui::LAYER_NOT_DRAWN);
   view->layer()->SetFillsBoundsOpaquely(false);
 }