(Merge to M-51) Revise the shelf alignment locking mechanism.

ash/shelf/shelf_types.h

 // Copyright (c) 2012 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_SHELF_SHELF_TYPES_H_
 #define ASH_SHELF_SHELF_TYPES_H_
 
 namespace ash {
 
 enum ShelfAlignment {
   SHELF_ALIGNMENT_BOTTOM,
   SHELF_ALIGNMENT_LEFT,
   SHELF_ALIGNMENT_RIGHT,
   // Top has never been supported.
+
+  // The locked alignment is set temporarily and not saved to preferences.
+  SHELF_ALIGNMENT_BOTTOM_LOCKED,
 };
 
 enum ShelfAutoHideBehavior {
   // Always auto-hide.
   SHELF_AUTO_HIDE_BEHAVIOR_ALWAYS,
 
   // Never auto-hide.
   SHELF_AUTO_HIDE_BEHAVIOR_NEVER,
 
   // Always hide.
@@ skipping 23 matching lines @@
   // The background when a window is overlapping.
   SHELF_BACKGROUND_OVERLAP,
 
   // The background when a window is maximized.
   SHELF_BACKGROUND_MAXIMIZED,
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELF_SHELF_TYPES_H_