Remove non-MD code from TrayBackgroundView

ash/common/system/overview/overview_button_tray.cc

Index: ash/common/system/overview/overview_button_tray.cc
diff --git a/ash/common/system/overview/overview_button_tray.cc b/ash/common/system/overview/overview_button_tray.cc
index 920948bb984b6740a3bfbb24361acc2883a1f525..2b7d243d2e182e975f610dda40cf126905129356 100644
--- a/ash/common/system/overview/overview_button_tray.cc
+++ b/ash/common/system/overview/overview_button_tray.cc
@@ -22,9 +22,8 @@
 namespace ash {
 
 OverviewButtonTray::OverviewButtonTray(WmShelf* wm_shelf)
-    : TrayBackgroundView(wm_shelf), icon_(new views::ImageView()) {
+    : TrayBackgroundView(wm_shelf, true), icon_(new views::ImageView()) {
   SetInkDropMode(InkDropMode::ON);
-  SetContentsBackground(false);
 
   icon_->SetImage(CreateVectorIcon(kShelfOverviewIcon, kShelfIconColor));
   SetIconBorderForShelfAlignment();