Move ReadFileToString to the base namespace.

chrome/browser/chromeos/drive/file_system/truncate_operation_unittest.cc

 // Copyright 2013 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/chromeos/drive/file_system/truncate_operation.h"
 
 #include "base/file_util.h"
 #include "base/files/file_path.h"
 #include "chrome/browser/chromeos/drive/drive.pb.h"
 #include "chrome/browser/chromeos/drive/fake_free_disk_space_getter.h"
@@ skipping 96 matching lines @@
   base::FilePath local_path;
   error = FILE_ERROR_FAILED;
   cache()->GetFileOnUIThread(
       GetLocalId(file_in_root),
       google_apis::test_util::CreateCopyResultCallback(&error, &local_path));
   test_util::RunBlockingPoolTask();
   ASSERT_EQ(FILE_ERROR_OK, error);
 
   // The local file should be truncated.
   std::string content;
-  ASSERT_TRUE(file_util::ReadFileToString(local_path, &content));
+  ASSERT_TRUE(base::ReadFileToString(local_path, &content));
 
   EXPECT_EQ(file_size + 10, static_cast<int64>(content.size()));
   // All trailing 10 bytes should be '\0'.
   EXPECT_EQ(std::string(10, '\0'), content.substr(file_size));
 }
 
 }  // namespace file_system
 }  // namespace drive