Use GetSystemSalt from the cryptohome lib instead of reading salt off disk.

chrome/browser/chromeos/cros/mock_cryptohome_library.h

 // Copyright (c) 2010 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_CROS_MOCK_CRYPTOHOME_LIBRARY_H_
 #define CHROME_BROWSER_CHROMEOS_CROS_MOCK_CRYPTOHOME_LIBRARY_H_
 
 #include <string>
 
 #include "chrome/browser/chromeos/cros/cryptohome_library.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
 namespace chromeos {
 
 class MockCryptohomeLibrary : public CryptohomeLibrary {
  public:
   MockCryptohomeLibrary() {}
   virtual ~MockCryptohomeLibrary() {}
   MOCK_METHOD2(Mount, bool(const std::string& user_email,
                            const std::string& passhash));
   MOCK_METHOD2(CheckKey, bool(const std::string& user_email,
                               const std::string& passhash));
   MOCK_METHOD0(IsMounted, bool(void));
+  MOCK_METHOD0(GetSystemSalt, CryptohomeBlob(void));
 };
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_CROS_MOCK_CRYPTOHOME_LIBRARY_H_