LibWeb: Added get source functionality and hook event

Author: https://github.com/xeons
Commit: 2f56a86a4e
Pull-request: https://github.com/SerenityOS/serenity/pull/5477
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/WebViewHooks.h

@@ -46,6 +46,7 @@ public:
     Function<void(const Gfx::Bitmap&)> on_favicon_change;
     Function<void(const URL&)> on_url_drop;
     Function<void(DOM::Document*)> on_set_document;
+    Function<void(const URL&, const String&)> on_get_source;
 };
 
 }