Remove non-MD code from TrayBackgroundView

ash/common/system/chromeos/palette/palette_tray.cc

Index: ash/common/system/chromeos/palette/palette_tray.cc
diff --git a/ash/common/system/chromeos/palette/palette_tray.cc b/ash/common/system/chromeos/palette/palette_tray.cc
index eccef71de33a8b82072ccb4f72070538ec570b83..94d3386bc6bdf5a669116c3917f328815bc7cb3a 100644
--- a/ash/common/system/chromeos/palette/palette_tray.cc
+++ b/ash/common/system/chromeos/palette/palette_tray.cc
@@ -151,17 +151,13 @@ class TitleView : public views::View, public views::ButtonListener {
 }  // namespace
 
 PaletteTray::PaletteTray(WmShelf* wm_shelf)
-    : TrayBackgroundView(wm_shelf),
+    : TrayBackgroundView(wm_shelf, true),
       palette_tool_manager_(new PaletteToolManager(this)),
       weak_factory_(this) {
   PaletteTool::RegisterToolInstances(palette_tool_manager_.get());
 
-  if (MaterialDesignController::IsShelfMaterial()) {
+  if (MaterialDesignController::IsShelfMaterial())
     SetInkDropMode(InkDropMode::ON);
-    SetContentsBackground(false);
-  } else {
-    SetContentsBackground(true);
-  }
 
   SetLayoutManager(new views::FillLayout());
   icon_ = new views::ImageView();