LibWeb+WebContent: Port DumpLayoutTree to OutOfProcessWebView

This required adding a simple OOPWV::dump_layout_tree() API that
synchronously requests a dump of the layout tree from the WebContent
process.

Userland/Services/WebContent/WebContentServer.ipc

@@ -34,6 +34,8 @@ endpoint WebContentServer
 
     run_javascript(String js_source) =|
 
+    dump_layout_tree() => (String dump)
+
     get_selected_text() => (String selection)
     select_all() =|
 }