Save uploaded files in multipart request to disk

Instead of keeping them in memory. Pass the file descriptor/file stream
to the handler process when asking for the uploaded file data.

BUG: 24166746
Change-Id: Ife4f5b4fa422d99272b15f6f02e4b3b515e4e3b4
diff --git a/libwebserv/request.cc b/libwebserv/request.cc
index cb0fa1b..7b518a4 100644
--- a/libwebserv/request.cc
+++ b/libwebserv/request.cc
@@ -36,8 +36,8 @@
 }
 
 void FileInfo::GetData(
-    const base::Callback<void(const std::vector<uint8_t>&)>& success_callback,
-    const base::Callback<void(chromeos::Error*)>& error_callback) {
+    const base::Callback<void(chromeos::StreamPtr)>& success_callback,
+    const base::Callback<void(chromeos::Error*)>& error_callback) const {
   handler_->GetFileData(request_id_,
                         file_id_,
                         success_callback,