chromium/chrome/test/data/extensions/api_test/browser_action/rect_icon/manifest.json

{
  "name": "Rectangle icons",
  "description": "A browser action with rectangular icons",
  "manifest_version": 2,
  "background": {
    "scripts": ["background.js"],
    "persistent": true
  },
  "browser_action": {
    "default_icon": "square.png"
  },
  "version": "1.0"
}