chromeos: Cleanup class/struct fwd declarations

chromeos/network/proxy/proxy_config_handler.h

Index: chromeos/network/proxy/proxy_config_handler.h
diff --git a/chromeos/network/proxy/proxy_config_handler.h b/chromeos/network/proxy/proxy_config_handler.h
index 124c1ba5951ce870e813310e280473ba9d6c67f2..d387d2dcf6ff4d138d947522d3fcc4ee2b69037a 100644
--- a/chromeos/network/proxy/proxy_config_handler.h
+++ b/chromeos/network/proxy/proxy_config_handler.h
@@ -10,14 +10,9 @@
 #include "chromeos/chromeos_export.h"
 #include "components/onc/onc_constants.h"
 
-class PrefRegistrySimple;
 class PrefService;
 class ProxyConfigDictionary;
 
-namespace user_prefs {
-class PrefRegistrySyncable;
-}
-
 namespace chromeos {
 
 class NetworkState;