Componentize more signin code

components/signin/core/browser/account_tracker_service.h

Index: components/signin/core/browser/account_tracker_service.h
diff --git a/components/signin/core/browser/account_tracker_service.h b/components/signin/core/browser/account_tracker_service.h
index 41cdd918c4abd40a6c1729ad5c58cf45622642e8..0c0c286a8f2302b4b2cc6678e22eace2b6ffbe80 100644
--- a/components/signin/core/browser/account_tracker_service.h
+++ b/components/signin/core/browser/account_tracker_service.h
@@ -25,6 +25,10 @@ namespace base {
 class DictionaryValue;
 }
 
+namespace user_prefs {
+class PrefRegistrySyncable;
+}
+
 // AccountTrackerService is a KeyedService that retrieves and caches GAIA
 // information about Google Accounts.
 class AccountTrackerService : public KeyedService,
@@ -80,6 +84,9 @@ class AccountTrackerService : public KeyedService,
   AccountTrackerService();
   ~AccountTrackerService() override;
 
+  // Registers the preferences used by AccountTrackerService.
+  static void RegisterPrefs(user_prefs::PrefRegistrySyncable* registry);
+
   // KeyedService implementation.
   void Shutdown() override;