Make FileStream::Seek async and add FileStream::SeekSync for sync operation

net/url_request/url_request_file_job.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.
 
 // For loading files, we make use of overlapped i/o to ensure that reading from
 // the filesystem (e.g., a network filesystem) does not block the calling
 // thread.  An alternative approach would be to use a background thread or pool
 // of threads, but it seems better to leverage the operating system's ability
 // to do background file reads for us.
 //
@@ skipping 322 matching lines @@
   DCHECK_GE(remaining_bytes_, 0);
 
   // URL requests should not block on the disk!
   //   http://code.google.com/p/chromium/issues/detail?id=59849
   {
     base::ThreadRestrictions::ScopedAllowIO allow_io;
     // Do the seek at the beginning of the request.
     if (remaining_bytes_ > 0 &&
         byte_range_.first_byte_position() != 0 &&
         byte_range_.first_byte_position() !=
-        stream_.Seek(FROM_BEGIN, byte_range_.first_byte_position())) {
+        stream_.SeekSync(FROM_BEGIN, byte_range_.first_byte_position())) {
       NotifyDone(URLRequestStatus(URLRequestStatus::FAILED,
                                   ERR_REQUEST_RANGE_NOT_SATISFIABLE));
       return;
     }
   }
 
   set_expected_content_size(remaining_bytes_);
   NotifyHeadersComplete();
 }
 
 void URLRequestFileJob::DidRead(int result) {
   if (result > 0) {
     SetStatus(URLRequestStatus());  // Clear the IO_PENDING status
   } else if (result == 0) {
     NotifyDone(URLRequestStatus());
   } else {
     NotifyDone(URLRequestStatus(URLRequestStatus::FAILED, result));
   }
 
   remaining_bytes_ -= result;
   DCHECK_GE(remaining_bytes_, 0);
 
   NotifyReadComplete(result);
 }
 
 }  // namespace net