Implement LoginDatabase on all platforms.

chrome/browser/password_manager/login_database_mac.cc

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "chrome/browser/password_manager/login_database_mac.h"
+#include "chrome/browser/password_manager/login_database.h"
 
-std::string LoginDatabaseMac::EncryptedString(const string16& plain_text)
-    const {
+// On the Mac, the LoginDatabase nulls out passwords, so that we can use the
+// rest of the database as a suplemental storage system to complement Keychain,
+// providing storage of fields Keychain doesn't allow.
+
+std::string LoginDatabase::EncryptedString(const string16& plain_text) const {
   return std::string();
 }
 
-string16 LoginDatabaseMac::DecryptedString(const std::string& cipher_text)
-    const {
+string16 LoginDatabase::DecryptedString(const std::string& cipher_text) const {
   return string16();
 }