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

net/base/file_stream_context_win.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 <windows.h>
 
 #include "base/files/file_path.h"
 #include "base/logging.h"
@@ skipping 44 matching lines @@
       async_in_progress_(false),
       orphaned_(false),
       task_runner_(task_runner),
       async_read_initiated_(false),
       async_read_completed_(false),
       io_complete_for_read_received_(false),
       result_(0) {
   io_context_.handler = this;
   memset(&io_context_.overlapped, 0, sizeof(io_context_.overlapped));
   if (file_.IsValid()) {
-    // TODO(hashimoto): Check that file_ is async.
+    DCHECK(file_.async());
     OnFileOpened();
   }
 }
 
 FileStream::Context::~Context() {
 }
 
 int FileStream::Context::Read(IOBuffer* buf,
                               int buf_len,
                               const CompletionCallback& callback) {
@@ skipping 31 matching lines @@
       IOCompletionIsPending(callback, buf);
     else
       LOG(WARNING) << "WriteFile failed: " << error.os_error;
     return static_cast<int>(error.result);
   }
 
   IOCompletionIsPending(callback, buf);
   return ERR_IO_PENDING;
 }
 
-FileStream::Context::IOResult FileStream::Context::SeekFileImpl(
-    base::File::Whence whence,
-    int64 offset) {
+FileStream::Context::IOResult FileStream::Context::SeekFileImpl(int64 offset) {
   LARGE_INTEGER result;
-  result.QuadPart = file_.Seek(whence, offset);
-  if (result.QuadPart >= 0) {
-    SetOffset(&io_context_.overlapped, result);
-    return IOResult(result.QuadPart, 0);
-  }
-
-  return IOResult::FromOSError(GetLastError());
+  result.QuadPart = offset;
+  SetOffset(&io_context_.overlapped, result);
+  return IOResult(result.QuadPart, 0);
 }
 
 void FileStream::Context::OnFileOpened() {
   base::MessageLoopForIO::current()->RegisterIOHandler(file_.GetPlatformFile(),
                                                        this);
 }
 
 void FileStream::Context::IOCompletionIsPending(
     const CompletionCallback& callback,
     IOBuffer* buf) {
@@ skipping 103 matching lines @@
 
   IOResult error = IOResult::FromOSError(os_error);
   if (error.os_error == ERROR_IO_PENDING) {
     InvokeUserCallback();
   } else {
     OnIOCompleted(&io_context_, 0, error.os_error);
   }
 }
 
 }  // namespace net