LibHTML: Store the HTML parser input along with the created Document

This will allow us to "view source" later on, long after parsing has
finished and turned it into a DOM.

Libraries/LibHTML/Parser/HTMLParser.cpp

@@ -334,6 +334,7 @@ RefPtr<Document> parse_html_document(const StringView& html, const URL& url)
 {
     auto document = adopt(*new Document);
     document->set_url(url);
+    document->set_source(html);
 
     if (!parse_html_document(html, *document, *document))
         return nullptr;