Chrome extension: How to open a link in new tab?

The problem is that you are violating manifest version 2’s content security policy. To fix it all you have to do is get rid of inline script, in this case your background page. Turn it into a background script like this:

manifest.json

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

background.js

chrome.browserAction.onClicked.addListener(function(activeTab){
  var newURL = "http://stackoverflow.com/";
  chrome.tabs.create({ url: newURL });
});

If, for some reason, you do need it to be a page, then simply include the script as an external file and declare it as a page like before.