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

storage/browser/fileapi/local_file_stream_writer.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 "storage/browser/fileapi/local_file_stream_writer.h"
 
 #include "base/message_loop/message_loop.h"
 #include "net/base/file_stream.h"
 #include "net/base/io_buffer.h"
 #include "net/base/net_errors.h"
@@ skipping 134 matching lines @@
     const base::Closure& main_operation) {
   DCHECK(has_pending_operation_);
   DCHECK(stream_impl_.get());
 
   if (initial_offset_ == 0) {
     // No need to seek.
     main_operation.Run();
     return;
   }
 
-  int result = stream_impl_->Seek(base::File::FROM_BEGIN, initial_offset_,
-                                  base::Bind(&LocalFileStreamWriter::DidSeek,
-                                             weak_factory_.GetWeakPtr(),
-                                             error_callback,
-                                             main_operation));
+  int result = stream_impl_->Seek(
+      initial_offset_,
+      base::Bind(&LocalFileStreamWriter::DidSeek, weak_factory_.GetWeakPtr(),
+                 error_callback, main_operation));
   if (result != net::ERR_IO_PENDING) {
     has_pending_operation_ = false;
     error_callback.Run(result);
   }
 }
 
 void LocalFileStreamWriter::DidSeek(
     const net::CompletionCallback& error_callback,
     const base::Closure& main_operation,
     int64 result) {
@@ skipping 77 matching lines @@
     return false;
 
   net::CompletionCallback pending_cancel = cancel_callback_;
   has_pending_operation_ = false;
   cancel_callback_.Reset();
   pending_cancel.Run(net::OK);
   return true;
 }
 
 }  // namespace storage