cryptohome: Add dbus method and signal name for encryption migration.

To Chromium, this adds

  HomedirMethods::MigrateToExt4Crypto(const Identification& id,
                                      const Authorization& auth,
                                      const Callback& callback);

that requests Cryptohomed to do the migration (for the |id|
and |auth| combination containing the same data that are passed
to MountEx), and

      const base::Calback<void(uint64_t, uint64_t)>&);

for listening to a periodic progress status update.

Depends on the constants definition in


Cr-Commit-Position: refs/heads/master@{#458268}
9 files changed