Remove non-MD code from TrayBackgroundView

ash/common/system/tray/system_tray.cc

Index: ash/common/system/tray/system_tray.cc
diff --git a/ash/common/system/tray/system_tray.cc b/ash/common/system/tray/system_tray.cc
index ba1381431294b4db073e463c5f3f3ecf8a1a42e2..ba1439136675f67f1da0baa874c9b07075afb4f0 100644
--- a/ash/common/system/tray/system_tray.cc
+++ b/ash/common/system/tray/system_tray.cc
@@ -219,15 +219,13 @@ SystemTray::SystemTray(WmShelf* wm_shelf)
       screen_share_tray_item_(nullptr) {
   if (MaterialDesignController::IsShelfMaterial()) {
     SetInkDropMode(InkDropMode::ON);
-    SetContentsBackground(false);
 
     // Since user avatar is on the right hand side of System tray of a
     // horizontal shelf and that is sufficient to indicate separation, no
     // separator is required.
     set_separator_visibility(false);
-  } else {
-    SetContentsBackground(true);
   }
+  SetContentsBackground();
 }
 
 SystemTray::~SystemTray() {