Introduce a type of View background that stays in sync with its host

ash/system/tray/tray_constants.h

 // 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.
 
 #ifndef ASH_SYSTEM_TRAY_TRAY_CONSTANTS_H_
 #define ASH_SYSTEM_TRAY_TRAY_CONSTANTS_H_
 
 #include "ash/ash_export.h"
 
 typedef unsigned int SkColor;
@@ skipping 56 matching lines @@
 extern const int kTrayPopupLabelRightPadding;
 
 extern const int kTrayPopupDetailsIconWidth;
 extern const int kTrayPopupDetailsLabelExtraLeftMargin;
 extern const SkColor kTrayPopupHoverBackgroundColor;
 extern const int kTrayRoundedBorderRadius;
 
 // The width of ToggleButton views including any border padding.
 extern const int kTrayToggleButtonWidth;
 
-extern const SkColor kBackgroundColor;
 extern const SkColor kPublicAccountUserCardTextColor;
 extern const SkColor kPublicAccountUserCardNameColor;
 
 extern const SkColor kHeaderBackgroundColor;
 
 extern const SkColor kHeaderTextColorNormal;
 extern const SkColor kHeaderTextColorHover;
 
 extern const int kTrayPopupMinWidth;
 extern const int kTrayPopupMaxWidth;
@@ skipping 54 matching lines @@
 extern const int kTrayPopupSystemInfoRowHeight;
 
 namespace test {
 const int kSettingsTrayItemViewId = 10000;
 const int kAccessibilityTrayItemViewId = 10001;
 }  // namespace test
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_TRAY_CONSTANTS_H_