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

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index 6f6892a3a9c5708e4e8bd6d62f1f229f81e02d01..bd8f96f28487015964dd04f3b3e4693ef2f9ece4 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -496,7 +496,6 @@ void SystemTray::ShowItems(const std::vector<SystemTrayItem*>& items,
       // This is the case where a volume control or brightness control bubble
       // is created.
       init_params.max_height = default_bubble_height_;
-      init_params.bg_color = kBackgroundColor;
     } else {
       init_params.bg_color = kHeaderBackgroundColor;
     }