[Sync] Removing duplicate includes, part 2.

components/sync/driver/system_encryptor_unittest.cc

 // Copyright 2014 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 "components/sync/driver/system_encryptor.h"
 
-#include <string>
-
 #include "build/build_config.h"
 #include "components/os_crypt/os_crypt_mocker.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 
 namespace {
 
 const char kPlaintext[] = "The Magic Words are Squeamish Ossifrage";
 
@@ skipping 11 matching lines @@
   EXPECT_NE(kPlaintext, ciphertext);
   std::string plaintext;
   EXPECT_TRUE(encryptor_.DecryptString(ciphertext, &plaintext));
   EXPECT_EQ(kPlaintext, plaintext);
   ::OSCryptMocker::TearDown();
 }
 
 }  // namespace
 
 }  // namespace syncer