Examine source code of viewerCanPlay

{
  "manifest_version": 2,
  "name": "viewerCanPlay",
  "version": "1.2",
  "description": "Adds quicker functionality to media pages",
  "icons": {
    "48": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "js": [
        "playin.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{1f43bdb9-72dd-417c-80c2-01dd461851dd}"
    }
  }
}