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

net/base/file_stream_context_posix.cc

 // 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.
 
 #include "net/base/file_stream_context.h"
 
 #include <errno.h>
 
 #include "base/basictypes.h"
 #include "base/bind.h"
@@ skipping 61 matching lines @@
       base::Bind(&Context::OnAsyncCompleted,
                  base::Unretained(this),
                  IntToInt64(callback)));
   DCHECK(posted);
 
   async_in_progress_ = true;
   return ERR_IO_PENDING;
 }
 
 FileStream::Context::IOResult FileStream::Context::SeekFileImpl(
-    base::File::Whence whence,
     int64_t offset) {
-  int64_t res = file_.Seek(whence, offset);
+  int64_t res = file_.Seek(base::File::FROM_BEGIN, offset);
   if (res == -1)
     return IOResult::FromOSError(errno);
 
   return IOResult(res, 0);
 }
 
 void FileStream::Context::OnFileOpened() {
 }
 
 FileStream::Context::IOResult FileStream::Context::ReadFileImpl(
@@ skipping 14 matching lines @@
     scoped_refptr<IOBuffer> buf,
     int buf_len) {
   int res = file_.WriteAtCurrentPosNoBestEffort(buf->data(), buf_len);
   if (res == -1)
     return IOResult::FromOSError(errno);
 
   return IOResult(res, 0);
 }
 
 }  // namespace net