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/Libraries/LibWeb/OutOfProcessWebView.cpp

@@ -460,4 +460,9 @@ void OutOfProcessWebView::select_all()
     client().async_select_all();
 }
 
+String OutOfProcessWebView::dump_layout_tree()
+{
+    return client().dump_layout_tree();
+}
+
 }