Remove non-MD code from TrayBackgroundView

ash/common/system/web_notification/web_notification_tray.cc

Index: ash/common/system/web_notification/web_notification_tray.cc
diff --git a/ash/common/system/web_notification/web_notification_tray.cc b/ash/common/system/web_notification/web_notification_tray.cc
index 6a2470d36284e35b68d9455f8eff36f17fdb1871..2d0d9bbfb712e14b329b2878cf5ee67ee2c3fd37 100644
--- a/ash/common/system/web_notification/web_notification_tray.cc
+++ b/ash/common/system/web_notification/web_notification_tray.cc
@@ -287,7 +287,7 @@ class WebNotificationLabel : public WebNotificationItem {
 WebNotificationTray::WebNotificationTray(WmShelf* shelf,
                                          WmWindow* status_area_window,
                                          SystemTray* system_tray)
-    : TrayBackgroundView(shelf),
+    : TrayBackgroundView(shelf, true),
       status_area_window_(status_area_window),
       system_tray_(system_tray),
       show_message_center_on_unlock_(false),
@@ -299,14 +299,12 @@ WebNotificationTray::WebNotificationTray(WmShelf* shelf,
 
   if (MaterialDesignController::IsShelfMaterial()) {
     SetInkDropMode(InkDropMode::ON);
-    SetContentsBackground(false);
     gfx::ImageSkia bell_image =
         CreateVectorIcon(kShelfNotificationsIcon, kShelfIconColor);
     const gfx::Size bell_icon_size = kTrayItemInnerIconSize;
     bell_icon_.reset(new WebNotificationImage(
         bell_image, bell_icon_size, animation_container_.get(), this));
   } else {
-    SetContentsBackground(true);
     gfx::ImageSkia bell_image =
         CreateVectorIcon(gfx::VectorIconId::NOTIFICATIONS, kNoUnreadIconSize,
                          kWebNotificationColorNoUnread);