Reuse NativeThemeAuraDark for cros tray.

ash/common/system/tray/tray_background_view.cc

Index: ash/common/system/tray/tray_background_view.cc
diff --git a/ash/common/system/tray/tray_background_view.cc b/ash/common/system/tray/tray_background_view.cc
index e6393764d72dda2bf251ca1f66e52c3359dffb96..63ce4c1f3e3bb9fb2125f48400b32ba03543c5a3 100644
--- a/ash/common/system/tray/tray_background_view.cc
+++ b/ash/common/system/tray/tray_background_view.cc
@@ -30,6 +30,7 @@
 #include "ui/gfx/nine_image_painter.h"
 #include "ui/gfx/skia_util.h"
 #include "ui/gfx/transform.h"
+#include "ui/native_theme/native_theme_dark_aura.h"
 #include "ui/views/background.h"
 #include "ui/views/layout/box_layout.h"
 #include "ui/wm/core/window_animations.h"
@@ -320,6 +321,10 @@ const char* TrayBackgroundView::GetClassName() const {
   return kViewClassName;
 }
 
+const ui::NativeTheme* TrayBackgroundView::GetNativeTheme() const {
+  return ui::NativeThemeDarkAura::instance();

[tdanderson, 2016/08/17 19:35:00]

I would prefer for this were MD-only, i.e.,

if (MaterialDesignController::IsShelfMaterial())
  return ui::NativeThemeDarkAura::instance();
return ActionableView::GetNativeTheme();

This guards against the possibility we accidentally regress something in non-MD
(not just for the system tray, but also for the other derived classes of
TrayBackgroundView).

[Evan Stade, 2016/08/18 00:38:02]

I don't think this is necessary and it's a little bit gross, but I will agree it
is safer (less likely to cause regressions), so Done.

+}
+
 void TrayBackgroundView::ChildPreferredSizeChanged(views::View* child) {
   PreferredSizeChanged();
 }