[ash-md] Fixes clipping of controls in system tray

ash/common/system/tray/tray_details_view.cc

Index: ash/common/system/tray/tray_details_view.cc
diff --git a/ash/common/system/tray/tray_details_view.cc b/ash/common/system/tray/tray_details_view.cc
index b73560965b855cab3e883bdd49e885d7f1991b11..9ca7fafd1f6a6578666e87fa161057232dcda5b3 100644
--- a/ash/common/system/tray/tray_details_view.cc
+++ b/ash/common/system/tray/tray_details_view.cc
@@ -322,6 +322,12 @@ void TrayDetailsView::CreateScrollableList() {
   scroll_content_ = new ScrollContentsView();
   scroller_ = new FixedSizedScrollView;
   scroller_->SetContentsView(scroll_content_);
+  // Make the |scroller_| have a layer to clip |scroll_content_|'s children.
+  // TODO(varkha): Make the sticky rows work with EnableViewPortLayer().
+  scroller_->SetPaintToLayer(true);
+  scroller_->set_background(
+      views::Background::CreateSolidBackground(kBackgroundColor));
+  scroller_->layer()->SetMasksToBounds(true);
 
   // Note: |scroller_| takes ownership of |scroll_border_|.
   if (!UseMd()) {