Force encryption migration if the device supports ARC.

chromeos/cryptohome/cryptohome_parameters.h

Index: chromeos/cryptohome/cryptohome_parameters.h
diff --git a/chromeos/cryptohome/cryptohome_parameters.h b/chromeos/cryptohome/cryptohome_parameters.h
index dd96100832ce6926388b86aba3e8b6d7b60636a2..7e60fc60c7968226b3693ea1be01a992d668ba4e 100644
--- a/chromeos/cryptohome/cryptohome_parameters.h
+++ b/chromeos/cryptohome/cryptohome_parameters.h
@@ -165,6 +165,11 @@ class CHROMEOS_EXPORT MountParameters {
 
   // If not empty, home dir will be created with these keys if it exist.
   std::vector<KeyDefinition> create_keys;
+
+  // If |true|, and cryptohomed supports the new "dircrypto" encryption,
+  // forces to use the new encryption. That is, makes it an error to mount
+  // an existing home directory encrypted in the old way (ecryptfs).
+  bool force_dircrypto_if_available = false;
 };
 
 // This function returns true if cryptohome of |account_id| is migrated to