Remove non-MD code from TrayBackgroundView

ash/common/system/tray/system_tray.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/common/system/tray/system_tray.h"
 
 #include <algorithm>
 #include <map>
 #include <vector>
 
@@ skipping 201 matching lines @@
       tray_cast_(nullptr),
       tray_date_(nullptr),
       tray_network_(nullptr),
       tray_tiles_(nullptr),
       tray_system_info_(nullptr),
       tray_update_(nullptr),
       screen_capture_tray_item_(nullptr),
       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() {
   // Destroy any child views that might have back pointers before ~View().
   activation_observer_.reset();
   key_event_watcher_.reset();
   system_bubble_.reset();
   notification_bubble_.reset();
   for (const auto& item : items_)
     item->DestroyTrayView();
@@ skipping 647 matching lines @@
           .work_area()
           .height();
   if (work_area_height > 0) {
     UMA_HISTOGRAM_CUSTOM_COUNTS(
         "Ash.SystemMenu.PercentageOfWorkAreaHeightCoveredByMenu",
         100 * bubble_view->height() / work_area_height, 1, 300, 100);
   }
 }
 
 }  // namespace ash