Examine source code of Active_flash

{
  "name": "flash",
  "version": "1.3",
  "description": "Modify the  while simulating a mobile viewport",
  "manifest_version": 2,
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "contentscript.js"
      ],
      "run_at": "document_start",
      "all_frames": true
    }
  ]
}