Add flag to track migration from email to gaia id.

components/signin/core/common/signin_pref_names.cc

Index: components/signin/core/common/signin_pref_names.cc
diff --git a/components/signin/core/common/signin_pref_names.cc b/components/signin/core/common/signin_pref_names.cc
index b5d72275bb79fca71d2a6de3814b3b901b4705d7..760a24564586600cd180c0d757134170f97003c2 100644
--- a/components/signin/core/common/signin_pref_names.cc
+++ b/components/signin/core/common/signin_pref_names.cc
@@ -6,6 +6,11 @@
 
 namespace prefs {
 
+// An integer property indicating the state of account id migration from
+// email to gaia id for the the profile.  See account_tracker_service.h
+// for possible values.
+const char kAccountIdMigrationState[] = "account_id_migration_state";
+
 // Boolean identifying whether reverse auto-login is enabled.
 const char kAutologinEnabled[] = "autologin.enabled";