LibWeb+WebContent: Do not reference-count file request objects

There is currently a memory leak with these file request objects due to
the callback on_file_request_finish referencing itself in its capture
list. This object does not need to be reference counted or allocated on
the heap. It is only ever stored in a HashMap until a response is
received from the browser, and it is not shared.

Userland/Libraries/LibWeb/Loader/FileRequest.cpp

@@ -8,8 +8,9 @@
 
 namespace Web {
 
-FileRequest::FileRequest(DeprecatedString path)
-    : m_path(move(path))
+FileRequest::FileRequest(DeprecatedString path, Function<void(ErrorOr<i32>)> on_file_request_finish_callback)
+    : on_file_request_finish(move(on_file_request_finish_callback))
+    , m_path(move(path))
 {
 }