Re-introduce the partial magnifier

ash/magnifier/magnifier_constants.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_MAGNIFIER_MAGNIFIER_CONSTANTS_H_
 #define ASH_MAGNIFIER_MAGNIFIER_CONSTANTS_H_
 
 namespace ash {
 
+// Note: Do not change these values; UMA and prefs depend on them.
 enum MagnifierType {
-  MAGNIFIER_OFF,
-  MAGNIFIER_FULL,
-  MAGNIFIER_PARTIAL,
+  MAGNIFIER_FULL    = 1,
+  MAGNIFIER_PARTIAL = 2,
 };
 
+const int kMaxMagnifierType = 2;
+
+const MagnifierType kDefaultMagnifierType = MAGNIFIER_FULL;
+
 }  // namespace ash
 
 #endif  // ASH_MAGNIFIER_MAGNIFIER_CONSTANTS_H_