Only support seeking file streams from the beginning of the file.

base/files/file.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.
 
 #ifndef BASE_FILES_FILE_H_
 #define BASE_FILES_FILE_H_
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 150 matching lines @@
 
   File();
 
   // Creates or opens the given file. This will fail with 'access denied' if the
   // |path| contains path traversal ('..') components.
   File(const FilePath& path, uint32 flags);
 
   // Takes ownership of |platform_file|.
   explicit File(PlatformFile platform_file);
 
+  // Takes ownership of |platform_file| and sets |async|.
+  File(PlatformFile platform_file, bool async);

[Lei Zhang, 2015/05/25 00:29:58]

Calling this with |async| set to false is the same as the ctor above. Maybe do
"static File CreateForAsyncHandle(PlatformFile platform_file)" instead?

[Reilly Grant (use Gerrit), 2015/05/26 23:41:33]

Done.

+
   // Creates an object with a specific error_details code.
   explicit File(Error error_details);
 
   // Move constructor for C++03 move emulation of this type.
   File(RValue other);
 
   ~File();
 
   // Move operator= for C++03 move emulation of this type.
   File& operator=(RValue other);
@@ skipping 194 matching lines @@
   FileTracing::ScopedEnabler trace_enabler_;
 
   Error error_details_;
   bool created_;
   bool async_;
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_