Update server-backed state key generation.

chromeos/system/statistics_provider.cc

Index: chromeos/system/statistics_provider.cc
diff --git a/chromeos/system/statistics_provider.cc b/chromeos/system/statistics_provider.cc
index 581629b3c10c0b9e57dd6d6764a294efb9eed39f..93557b9e65972f5184a0ed320f15a06e0eb3d8a0 100644
--- a/chromeos/system/statistics_provider.cc
+++ b/chromeos/system/statistics_provider.cc
@@ -72,6 +72,7 @@ const char kHardwareClassKey[] = "hardware_class";
 const char kOffersCouponCodeKey[] = "ubind_attribute";
 const char kOffersGroupCodeKey[] = "gbind_attribute";
 const char kRlzBrandCodeKey[] = "rlz_brand_code";
+const char kDiskSerialNumber[] = "root_disk_serial_number";
 
 // OEM specific statistics. Must be prefixed with "oem_".
 const char kOemCanExitEnterpriseEnrollmentKey[] = "oem_can_exit_enrollment";