[ash-md] Improves smoothness with many windows in overview

ash/common/wm/overview/scoped_transform_overview_window.cc

Index: ash/common/wm/overview/scoped_transform_overview_window.cc
diff --git a/ash/common/wm/overview/scoped_transform_overview_window.cc b/ash/common/wm/overview/scoped_transform_overview_window.cc
index a2abe5d0e1a77c9b1e202b0c0937221f80d6ad02..745d464a95cc80404fb1cb0efb0b65e17466f564 100644
--- a/ash/common/wm/overview/scoped_transform_overview_window.cc
+++ b/ash/common/wm/overview/scoped_transform_overview_window.cc
@@ -268,7 +268,7 @@ void ScopedTransformOverviewWindow::RestoreWindow() {
   ScopedAnimationSettings animation_settings_list;
   BeginScopedAnimation(OverviewAnimationType::OVERVIEW_ANIMATION_RESTORE_WINDOW,
                        &animation_settings_list);
-  SetTransform(window()->GetRootWindow(), original_transform_, 0);
+  SetTransform(window()->GetRootWindow(), original_transform_, false, 0);
 
   std::unique_ptr<ScopedOverviewAnimationSettings> animation_settings =
       CreateScopedOverviewAnimationSettings(
@@ -417,21 +417,23 @@ gfx::Transform ScopedTransformOverviewWindow::GetTransformForRect(
 void ScopedTransformOverviewWindow::SetTransform(
     WmWindow* root_window,
     const gfx::Transform& transform,
+    bool use_masks,
     float radius) {
   DCHECK(overview_started_);
 
   if (ash::MaterialDesignController::IsOverviewMaterial() &&
       &transform != &original_transform_) {
-    if (!mask_) {
+    if (use_masks && !mask_) {
       mask_.reset(new OverviewContentMask(radius));
       mask_->layer()->SetFillsBoundsOpaquely(false);
       window()->GetLayer()->SetMaskLayer(mask_->layer());
     }
     gfx::Rect bounds(GetTargetBoundsInScreen().size());
-    mask_->layer()->SetBounds(bounds);
-    mask_->set_radius(radius);
-    window()->GetLayer()->SchedulePaint(bounds);
-
+    if (mask_) {
+      mask_->layer()->SetBounds(bounds);
+      mask_->set_radius(radius);
+      window()->GetLayer()->SchedulePaint(bounds);
+    }
     if (!determined_original_window_shape_) {
       determined_original_window_shape_ = true;
       SkRegion* window_shape = window()->GetLayer()->alpha_shape();
@@ -439,7 +441,8 @@ void ScopedTransformOverviewWindow::SetTransform(
         original_window_shape_.reset(new SkRegion(*window_shape));
     }
     const int inset =
-        window()->GetIntProperty(WmWindowProperty::TOP_VIEW_INSET);
+        use_masks ? window()->GetIntProperty(WmWindowProperty::TOP_VIEW_INSET)
+                  : 0;
     if (inset > 0) {
       bounds.Inset(0, inset, 0, 0);
       SkRegion* region = new SkRegion;