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

net/base/file_stream_context_win.cc

Index: net/base/file_stream_context_win.cc
diff --git a/net/base/file_stream_context_win.cc b/net/base/file_stream_context_win.cc
index 1a39384c5ee66d3ab843e977aa5ccae36788ec0a..a64f4b2f4d1919d1b1d60b42ec83f78de64c5cd5 100644
--- a/net/base/file_stream_context_win.cc
+++ b/net/base/file_stream_context_win.cc
@@ -62,7 +62,7 @@ FileStream::Context::Context(base::File file,
   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();
   }
 }
@@ -114,17 +114,11 @@ int FileStream::Context::Write(IOBuffer* 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() {