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

ash/system/audio/volume_view.cc

Index: ash/system/audio/volume_view.cc
diff --git a/ash/system/audio/volume_view.cc b/ash/system/audio/volume_view.cc
index efbdd2f7ac7be59724f3245afff01cb4701813b7..c7ed0f35677836ca10b9f05a0bc48a34735d0fbd 100644
--- a/ash/system/audio/volume_view.cc
+++ b/ash/system/audio/volume_view.cc
@@ -20,6 +20,7 @@
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/paint_vector_icon.h"
 #include "ui/gfx/vector_icon_types.h"
+#include "ui/native_theme/native_theme.h"
 #include "ui/views/background.h"
 #include "ui/views/border.h"
 #include "ui/views/controls/button/custom_button.h"
@@ -130,7 +131,9 @@ VolumeView::VolumeView(SystemTrayItem* owner,
       l10n_util::GetStringUTF16(IDS_ASH_STATUS_TRAY_VOLUME));
   tri_view_->AddView(TriView::Container::CENTER, slider_);
 
-  set_background(views::Background::CreateSolidBackground(kBackgroundColor));
+  set_background(views::Background::CreateThemedSolidBackground(
+      this, ui::NativeTheme::kColorId_BubbleBackground));
+
   Update();
 }