Remove all code related to the PreRead field trial.

chrome/app/file_pre_reader_win.h

 // Copyright (c) 2012 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.
 
 // This file defines a function to pre-read a file in order to avoid touching
 // the disk when it is subsequently used.
 
 #ifndef CHROME_APP_FILE_PRE_READER_WIN_H_
 #define CHROME_APP_FILE_PRE_READER_WIN_H_
 
 namespace base {
 class FilePath;
 }
 
-namespace startup_metric_utils {
-struct PreReadOptions;
-}
-
 // Pre-reads |file_path| to avoid touching the disk when the file is actually
-// used. Checks |pre_read_options| to determine how to pre-read the file.
-void PreReadFile(const base::FilePath& file_path,
-                 const startup_metric_utils::PreReadOptions& pre_read_options);
+// used.
+void PreReadFile(const base::FilePath& file_path);
 
 #endif  // CHROME_APP_FILE_PRE_READER_WIN_H_