Chrome extensions, injecting javascript on document_start

js-on-document_start

//In your manifest.json, this is sample code for running a content script on 'document_start'

"content_scripts": [ 
   { 
      "all_frames": false, 
      "js": [ "start.js", "main.js" ], 
      "matches": [ "*://*.domain.com/*" ], //over match urls, you can be more precise in start.js 
      "run_at": "document_start" 
   } 
],