Show small notification icons in notification tray (reland)

ash/common/system/tray/tray_background_view.h

Index: ash/common/system/tray/tray_background_view.h
diff --git a/ash/common/system/tray/tray_background_view.h b/ash/common/system/tray/tray_background_view.h
index 905a083cc86d6abb9895e8576d2b2ef253960cb8..741417400d92b36e94da1bf267be46326569fde2 100644
--- a/ash/common/system/tray/tray_background_view.h
+++ b/ash/common/system/tray/tray_background_view.h
@@ -43,6 +43,8 @@ class ASH_EXPORT TrayBackgroundView : public ActionableView,
 
     void set_size(const gfx::Size& size) { size_ = size; }
 
+    void SetMargin(const gfx::Insets& margin);
+
     // views::View:
     gfx::Size GetPreferredSize() const override;
 
@@ -58,6 +60,7 @@ class ASH_EXPORT TrayBackgroundView : public ActionableView,
 
     ShelfAlignment alignment_;
     gfx::Size size_;
+    gfx::Insets margin_;
 
     DISALLOW_COPY_AND_ASSIGN(TrayContainer);
   };