LibWebView+WebContent: Add IPC to run the fragment serialization steps

Author: https://github.com/trflynn89
Commit: 82af1557dd
Pull-request: https://github.com/SerenityOS/serenity/pull/15897
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -37,6 +37,7 @@ public:
 
     void debug_request(String const& request, String const& argument = {});
     void get_source();
+    String serialize_source();
 
     void inspect_dom_tree();
     struct DOMNodeProperties {