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

base/files/file.cc

 // Copyright (c) 2011 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 "base/files/file.h"
 #include "base/files/file_path.h"
 #include "base/files/file_tracing.h"
 #include "base/metrics/histogram.h"
 #include "base/timer/elapsed_timer.h"
 
@@ skipping 45 matching lines @@
       error_details_(other.object->error_details()),
       created_(other.object->created()),
       async_(other.object->async_) {
 }
 
 File::~File() {
   // Go through the AssertIOAllowed logic.
   Close();
 }
 
+// static
+File File::CreateForAsyncHandle(PlatformFile platform_file) {
+  File file(platform_file);
+  // It would be nice if we could validate that |platform_file| was opened
+  // with FLAG_OVERLAPPED on Windows but this doesn't appear to be possible.

[scottmg, 2015/06/15 21:42:45]

nit; FILE_FLAG_OVERLAPPED (I don't know of a direct way to check either)

[Reilly Grant (use Gerrit), 2015/06/15 22:01:12]

Done.

+  file.async_ = true;
+  return file;
+}
+
 File& File::operator=(RValue other) {
   if (this != other.object) {
     Close();
     SetPlatformFile(other.object->TakePlatformFile());
     path_ = other.object->path_;
     error_details_ = other.object->error_details();
     created_ = other.object->created();
     async_ = other.object->async_;
   }
   return *this;
@@ skipping 57 matching lines @@
 
 bool File::Flush() {
   ElapsedTimer timer;
   SCOPED_FILE_TRACE("Flush");
   bool return_value = DoFlush();
   UMA_HISTOGRAM_TIMES("PlatformFile.FlushTime", timer.Elapsed());
   return return_value;
 }
 
 }  // namespace base