Transform ImagePreReader into PreReadFile.

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_
+
+#include <stddef.h>
+
+#include "base/files/file_path.h"

[grt (UTC plus 2), 2015/11/03 04:02:15]

forward declare class FilePath
(http://www.chromium.org/developers/coding-style#TOC-Forward-Declarations-vs.-...)

[fdoray, 2015/11/03 21:23:39]

Done.

+
+// Reads the file passed in to avoid touching the disk when the file is actually

[grt (UTC plus 2), 2015/11/03 04:02:15]

nit: "Reads |file_path| to avoid..."

[fdoray, 2015/11/03 21:23:39]

Done.

+// used. The function checks the Windows version to determine which pre-reading
+// mechanism to use. On Vista+, chunks of |step_size| bytes are read into a
+// buffer. The bigger |step_size| is, the faster the file is pre-read (up to
+// about 4MB according to local tests), but also the more memory is allocated

[fdoray, 2015/10/30 21:08:54]

I plan to do a Canary experiment to find the best value. Once I know the best
value, I will remove the |step_size| arg and always use the best value. Note: On
a test laptop, files are pre-read ~25% faster when I use 4MB instead of 1MB (the
value that we currently use).

+// for the buffer.
+bool PreReadFile(const base::FilePath& file_path, size_t step_size);

[grt (UTC plus 2), 2015/11/03 04:02:15]

size_t -> int as per .cc file

[fdoray, 2015/11/03 21:23:39]

Done.

+
+#endif  // CHROME_APP_FILE_PRE_READER_WIN_H_