Examine source code of Silent Alarm

{
  "manifest_version": 2,
  "name": "Silent Alarm",
  "version": "1.0",
  "description": "Silent alarm using visual flash and vibration instead of sound.",
  "permissions": [
    "alarms",
    "notifications"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "silent-alarm@amo.local",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  }
}