Add an option to show debug fragment borders

This is quite a bit cleaner than abusing the rust debug functionality.
If we start collecting too many debugging options in the servo
executable we could opt to organize them into a single option.

Fixes #2263.

ports/cef/core.rs

@@ -67,6 +67,7 @@ pub extern "C" fn cef_run_message_loop() {
         hard_fail: false,
         bubble_inline_sizes_separately: false,
         show_debug_borders: false,
+        show_debug_fragment_borders: false,
         enable_text_antialiasing: true,
         trace_layout: false,
         devtools_port: None,