add back iframe (without src) support

this makes inspector work again for https://ember-twiddle.com/

Author: patricklx

skeletons/web-extension/manifest.json

@@ -26,13 +26,15 @@
       "world": "MAIN",
       "run_at": "document_start",
       "match_about_blank": true,
+      "match_origin_as_fallback": true,
       "all_frames": true
     },
     {
       "matches": ["<all_urls>"],
       "js": ["content-script.js"],
       "run_at": "document_end",
       "match_about_blank": true,
+      "match_origin_as_fallback": true,
       "all_frames": true
     }
   ],