How to inject custom JavaScript in web page? (simple google chrome plugin)

injected.js

// Your code

injector.js

var script = document.createElement('script');
script.setAttribute('type', 'text/javascript');
script.src = chrome.extension.getURL('injected.js');
setTimeout(function() {
	document.head.appendChild(script);
});

manifest.json

{
    "manifest_version": 2,
    "name": "Inject custom script",
    "short_name": "InjectCustomScript",
    "version": "1.0.0",
    "content_scripts": [{
		"matches": [
			"*://*/*"
		],
        "js": [ "injector.js" ],
        "run_at": "document_start",
		"persistent": false
    }],
    "web_accessible_resources": [
        "injected.js"
    ]
}