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/Services/WebContent/WebContentServer.ipc

@@ -32,6 +32,7 @@ endpoint WebContentServer
 
     debug_request(String request, String argument) =|
     get_source() =|
+    serialize_source() => (String source)
     inspect_dom_tree() =|
     inspect_dom_node(i32 node_id, Optional<Web::CSS::Selector::PseudoElement> pseudo_element) => (bool has_style, String specified_style, String computed_style, String custom_properties, String node_box_sizing)
     get_hovered_node_id() => (i32 node_id)