2nd display should show the same background as login/lock screen:

ash/shell_window_ids.h

Index: ash/shell_window_ids.h
diff --git a/ash/shell_window_ids.h b/ash/shell_window_ids.h
index 695ed8df65bc76231f694c0954e51d2b2e8ba417..8ac0f188cf026325ce973a03782985465b3225db 100644
--- a/ash/shell_window_ids.h
+++ b/ash/shell_window_ids.h
@@ -57,28 +57,31 @@ const int kShellWindowId_SystemModalContainer = 10;
 // the AppList and SystemModal dialogs.
 const int kShellWindowId_InputMethodContainer = 11;
 
+// The container for the lock screen background.
+const int kShellWindowId_LockScreenBackgroundContainer = 12;
+
 // The container for the lock screen.
-const int kShellWindowId_LockScreenContainer = 12;
+const int kShellWindowId_LockScreenContainer = 13;
 
 // The container for the lock screen modal windows.
-const int kShellWindowId_LockSystemModalContainer = 13;
+const int kShellWindowId_LockSystemModalContainer = 14;
 
 // The container for the status area.
-const int kShellWindowId_StatusContainer = 14;
+const int kShellWindowId_StatusContainer = 15;
 
 // The container for menus.
-const int kShellWindowId_MenuContainer = 15;
+const int kShellWindowId_MenuContainer = 16;
 
 // The container for drag/drop images and tooltips.
-const int kShellWindowId_DragImageAndTooltipContainer = 16;
+const int kShellWindowId_DragImageAndTooltipContainer = 17;
 
 // The container for bubbles briefly overlaid onscreen to show settings changes
 // (volume, brightness, etc.).
-const int kShellWindowId_SettingBubbleContainer = 17;
+const int kShellWindowId_SettingBubbleContainer = 18;
 
 // The container for special components overlaid onscreen, such as the
 // region selector for partial screenshots.
-const int kShellWindowId_OverlayContainer = 18;
+const int kShellWindowId_OverlayContainer = 19;
 
 }  // namespace internal