Google Chrome / Firefox do not see extension output in console

I am trying to test sample code for web extension for browsers.
But, it doesn't work. I checked the console for Google Chrome and also for Firefox. It doesn't print anything. The following is my code:

manifest.json:

{
    "description": "Demonstrating webRequests",
    "manifest_version": 2,
    "name": "webRequest-demo",
    "version": "1.0",

    "permissions": [
        "webRequest"

    ],
    "background": {
        "scripts": ["background.js"]
    }
}

background.js:

function logURL(requestDetails) {

    console.log("Loading: " + requestDetails.url);
}

chrome.webRequest.onBeforeRequest.addListener(
    logURL,
    {urls: [""]}
);
console.log("Hell o extension background script executed");

Am I missing something?