LibWeb: Implement IDBOpenDBRequest.onblocked

Author: https://github.com/jamierocks
Commit: 30db7fb323
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/242
Reviewed-by: https://github.com/AtkinsSJ ✅

Userland/Libraries/LibWeb/IndexedDB/IDBOpenDBRequest.idl

@@ -4,6 +4,6 @@
 [Exposed=(Window,Worker)]
 interface IDBOpenDBRequest : IDBRequest {
     // Event handlers:
-    [FIXME] attribute EventHandler onblocked;
+    attribute EventHandler onblocked;
     [FIXME] attribute EventHandler onupgradeneeded;
 };