Examine source code of FPSCounter

{
  "manifest_version": 2,
  "name": "FPSCounter",
  "version": "0.2.3",
  "description": "Injects a FPS counter into every site containing a canvas.",
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "run_at": "document_end",
      "js": [
        "fpscounter.js"
      ]
    }
  ]
}