[Sync] Don't require keystore migration time to be set in nigori node

sync/internal_api/sync_encryption_handler_impl.cc

Index: sync/internal_api/sync_encryption_handler_impl.cc
diff --git a/sync/internal_api/sync_encryption_handler_impl.cc b/sync/internal_api/sync_encryption_handler_impl.cc
index f83c3e1a282b13e0ac685a44b32da50e5a31e274..2d2b0cff7d951e0eb911af16b1524aed23eda614 100644
--- a/sync/internal_api/sync_encryption_handler_impl.cc
+++ b/sync/internal_api/sync_encryption_handler_impl.cc
@@ -70,15 +70,12 @@ enum NigoriMigrationState {
 // The new passphrase state is sufficient to determine whether a nigori node
 // is migrated to support keystore encryption. In addition though, we also
 // want to verify the conditions for proper keystore encryption functionality.
-// 1. Passphrase state is set.
-// 2. Migration time is set.
-// 3. Frozen keybag is true
-// 4. If passphrase state is keystore, keystore_decryptor_token is set.
+// 1. Passphrase type is set.
+// 2. Frozen keybag is true
+// 3. If passphrase state is keystore, keystore_decryptor_token is set.
 bool IsNigoriMigratedToKeystore(const sync_pb::NigoriSpecifics& nigori) {
   if (!nigori.has_passphrase_type())
     return false;
-  if (!nigori.has_keystore_migration_time())
-    return false;
   if (!nigori.keybag_is_frozen())
     return false;
   if (nigori.passphrase_type() ==
@@ -88,8 +85,6 @@ bool IsNigoriMigratedToKeystore(const sync_pb::NigoriSpecifics& nigori) {
           sync_pb::NigoriSpecifics::KEYSTORE_PASSPHRASE &&
       nigori.keystore_decryptor_token().blob().empty())
     return false;
-  if (!nigori.has_keystore_migration_time())
-    return false;
   return true;
 }
 
@@ -896,7 +891,6 @@ bool SyncEncryptionHandlerImpl::ApplyNigoriUpdateImpl(
   }
   bool is_nigori_migrated = IsNigoriMigratedToKeystore(nigori);
   if (is_nigori_migrated) {
-    DCHECK(nigori.has_keystore_migration_time());
     migration_time_ = ProtoTimeToTime(nigori.keystore_migration_time());
     PassphraseType nigori_passphrase_type =
         ProtoPassphraseTypeToEnum(nigori.passphrase_type());