Starts splitting out 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 {
+
+const int kShellWindowId_DockedContainer = 8;
+
+}  // namespace ash
+
+#endif  // ASH_WM_COMMON_WM_SHELL_WINDOW_IDS_H_