Moves DefaultContainer and PanelContainer to wm_shell_window_ids

ash/wm/common/wm_shell_window_ids.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_WM_COMMON_WM_SHELL_WINDOW_IDS_H_
 #define ASH_WM_COMMON_WM_SHELL_WINDOW_IDS_H_
 
 #include "ash/ash_export.h"
 
 // Defines the ids assigned to known containers. The id for a WmWindow is
 // accessed via GetShellWindowId().
 
 // TODO(sky): move this into wm namespace.
 namespace ash {
 
+// NOTE: while the value of these is not necessarily important, the ordering
+// is and is enforced in ash/shell_window_ids.h.
+
+// The container for standard top-level windows.
+const int kShellWindowId_DefaultContainer = 6;
+
+// The container for windows docked to either side of the desktop.
 const int kShellWindowId_DockedContainer = 8;
 
+// The container for panel windows.
+const int kShellWindowId_PanelContainer = 11;
+
 }  // namespace ash
 
 #endif  // ASH_WM_COMMON_WM_SHELL_WINDOW_IDS_H_