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

net/base/file_stream.cc

Index: net/base/file_stream.cc
diff --git a/net/base/file_stream.cc b/net/base/file_stream.cc
index 818fb2ee537f71150eb01d948450a140adaa6e19..1707fb755ec140ea653c42eff11ebe4834a95529 100644
--- a/net/base/file_stream.cc
+++ b/net/base/file_stream.cc
@@ -39,8 +39,13 @@ bool FileStream::IsOpen() const {
   return impl_.IsOpen();
 }
 
-int64 FileStream::Seek(Whence whence, int64 offset) {
-  return impl_.Seek(whence, offset);
+int FileStream::Seek(Whence whence, int64 offset,
+                     const Int64CompletionCallback& callback) {
+  return impl_.Seek(whence, offset, callback);
+}
+
+int64 FileStream::SeekSync(Whence whence, int64 offset) {
+  return impl_.SeekSync(whence, offset);
 }
 
 int64 FileStream::Available() {