Use only timezone IDs from known timezone list.

chrome/browser/chromeos/dom_ui/system_settings_provider.h

Index: chrome/browser/chromeos/dom_ui/system_settings_provider.h
diff --git a/chrome/browser/chromeos/dom_ui/system_settings_provider.h b/chrome/browser/chromeos/dom_ui/system_settings_provider.h
index 816ea2c926dff7cae09797810f4bf7ec55493835..a355df6c507892d8c07c8eceaa53f568eb961a74 100644
--- a/chrome/browser/chromeos/dom_ui/system_settings_provider.h
+++ b/chrome/browser/chromeos/dom_ui/system_settings_provider.h
@@ -46,6 +46,15 @@ class SystemSettingsProvider : public CrosSettingsProvider,
   // Gets timezone object from its id.
   const icu::TimeZone* GetTimezone(const string16& timezone_id);
 
+  // Gets a timezone id from a timezone in |timezones_| that has the same
+  // rule of given |timezone|.
+  // One timezone could have multiple timezones,
+  // e.g.
+  //   US/Pacific == America/Los_Angeles
+  // We should always use the known timezone id when passing back as
+  // pref values.
+  string16 GetKnownTimezoneID(const icu::TimeZone& timezone) const;
+
   // Timezones.
   std::vector<icu::TimeZone*> timezones_;