[Chrome OS MD] Draw a 1px separator between 2 tray items

ash/common/system/overview/overview_button_tray.h

Index: ash/common/system/overview/overview_button_tray.h
diff --git a/ash/common/system/overview/overview_button_tray.h b/ash/common/system/overview/overview_button_tray.h
index 1654458d62aba826889cd1926270efe619de7326..d74998333f8db79cbd636a83b736b119d6b9b6a6 100644
--- a/ash/common/system/overview/overview_button_tray.h
+++ b/ash/common/system/overview/overview_button_tray.h
@@ -16,6 +16,7 @@ class ImageView;
 }
 
 namespace ash {
+class ViewObserver;
 
 // Status area tray for showing a toggle for Overview Mode. Overview Mode
 // is equivalent to WindowSelectorController being in selection mode.
@@ -26,7 +27,7 @@ class ASH_EXPORT OverviewButtonTray : public TrayBackgroundView,
                                       public SessionStateObserver,
                                       public ShellObserver {
  public:
-  explicit OverviewButtonTray(WmShelf* wm_shelf);
+  explicit OverviewButtonTray(WmShelf* wm_shelf, ViewObserver* view_observer);
   ~OverviewButtonTray() override;
 
   // Updates the tray's visibility based on the LoginStatus and the current
@@ -57,6 +58,10 @@ class ASH_EXPORT OverviewButtonTray : public TrayBackgroundView,
   // alignment of the shelf.
   void SetIconBorderForShelfAlignment();
 
+  // Sets an empty border around the overview tray to change user-visible size
+  // of the dark background. Doesn't do anything in MD.
+  void SetOverviewTrayBorder();
+
   // Sets the icon to visible if maximize mode is enabled and
   // WindowSelectorController::CanSelect.
   void UpdateIconVisibility();