lsp-ui-doc no longer works with webkit

Rendering lsp-ui-doc in a client frame is perfectly fine, but when setting
```lsp-ui-doc-use-webkit``` to ```t```, emacs will spam this message:
```
/nix/store/5364q66m42jg3si8h82wq9v617xhnj7z-emacs-packages-deps/share/emacs/site-lisp/elpa/ls
p-ui-20190809.1907/lsp-ui-doc.html:30:24: TypeError: undefined is not an object (evaluating 'converter.makeHtml')
```
