Adds a new checkbox for backup and restore in opt-in flow.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 845e59327c09aeeb0ec0cf7bcc6243947807101a..92581dcb7ca02d0bedaff62328bc1763b4b1e082 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -16,6 +16,8 @@ namespace prefs {
 #if defined(OS_CHROMEOS) && defined(ENABLE_APP_LIST)
 // A preference to keep list of Android apps and their state.
 const char kArcApps[] = "arc.apps";
+// A preference to store backup and restore state for Android apps.
+const char kArcBackupRestoreEnabled[] = "arc.backup_restore_enabled";
 // A preference to keep Android apps enabled state.
 const char kArcEnabled[] = "arc.enabled";
 // A preference to keep list of Android packages and their infomation.