Re-introduce the partial magnifier

ash/magnifier/magnifier_constants.h

Index: ash/magnifier/magnifier_constants.h
diff --git a/ash/magnifier/magnifier_constants.h b/ash/magnifier/magnifier_constants.h
index 4ca4c892e97b3b01cd6f9e7039fa1e1906547a23..aa7cac72c014d3f77dbefe3e6a0f56fe4506838e 100644
--- a/ash/magnifier/magnifier_constants.h
+++ b/ash/magnifier/magnifier_constants.h
@@ -7,12 +7,16 @@
 
 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_