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 26 matching lines @@
 File::File(PlatformFile platform_file)
     : file_(platform_file),
       error_details_(FILE_OK),
       created_(false),
       async_(false) {
 #if defined(OS_POSIX)
   DCHECK_GE(platform_file, -1);
 #endif
 }
 
+File::File(PlatformFile platform_file, bool async)
+    : file_(platform_file),
+      error_details_(FILE_OK),
+      created_(false),
+      async_(async) {
+#if defined(OS_POSIX)
+  DCHECK_GE(platform_file, -1);
+#endif
+}
+
 File::File(Error error_details)
     : error_details_(error_details),
       created_(false),
       async_(false) {
 }
 
 File::File(RValue other)
     : file_(other.object->TakePlatformFile()),
       path_(other.object->path_),
       error_details_(other.object->error_details()),
@@ skipping 76 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