linux and chromeos: Turn on -Wunused-const-variable.

chrome/browser/chromeos/settings/device_settings_provider.cc

Index: chrome/browser/chromeos/settings/device_settings_provider.cc
diff --git a/chrome/browser/chromeos/settings/device_settings_provider.cc b/chrome/browser/chromeos/settings/device_settings_provider.cc
index a3366ae52a5c9eaf2d1b50d6c2bf3b6ded791fa3..7c1d3b894ebbca55cc025751ef00eafe2a5b40bb 100644
--- a/chrome/browser/chromeos/settings/device_settings_provider.cc
+++ b/chrome/browser/chromeos/settings/device_settings_provider.cc
@@ -82,9 +82,6 @@ const char* kKnownSettings[] = {
   kVariationsRestrictParameter,
 };
 
-// Legacy policy file location. Used to detect migration from pre v12 ChromeOS.
-const char kLegacyPolicyFile[] = "/var/lib/whitelist/preferences";
-
 bool HasOldMetricsFile() {
   // TODO(pastarmovj): Remove this once migration is not needed anymore.
   // If the value is not set we should try to migrate legacy consent file.