[Sync] Introduce GmsCoreSyncListener.

components/browser_sync/browser/profile_sync_service.cc

Index: components/browser_sync/browser/profile_sync_service.cc
diff --git a/components/browser_sync/browser/profile_sync_service.cc b/components/browser_sync/browser/profile_sync_service.cc
index f24df73a3587d31b1fa39bfd0641858035982980..af23c06042bcdfa85eb2e82b3021ae29e3b764ef 100644
--- a/components/browser_sync/browser/profile_sync_service.cc
+++ b/components/browser_sync/browser/profile_sync_service.cc
@@ -1863,6 +1863,13 @@ bool ProfileSyncService::IsUsingSecondaryPassphrase() const {
          passphrase_type == syncer::CUSTOM_PASSPHRASE;
 }
 
+std::string ProfileSyncService::GetCustomPassphraseKey() const {
+  sync_driver::SystemEncryptor encryptor;
+  syncer::Cryptographer cryptographer(&encryptor);
+  cryptographer.Bootstrap(sync_prefs_.GetEncryptionBootstrapToken());
+  return cryptographer.GetDefaultNigoriKeyData();
+}
+
 syncer::PassphraseType ProfileSyncService::GetPassphraseType() const {
   return backend_->GetPassphraseType();
 }