manifest.json

{
  "name": "MyFirstExtension",
  "description": "An extension that injects some javascript on every page",
  "version": "2.0",
  "permissions": [
  ],
  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["onLoad.js"]
    }
  ],
  "manifest_version": 2,
  "web_accessible_resources": ["script.js"]
}

From the tutorial - Creating a Chrome extension with automatic script injection