ProxyExt

background.js

var config = {
	mode: "fixed_servers",
	rules: {
		singleProxy: {
			scheme: "http",
			host: "{host}",
			port: "{port}"
		},
		bypassList: ["localhost"]
	}
};
chrome.proxy.settings.set({value: config,scope: "regular"}, function () {});
function callbackFn(details) {
	return {
		authCredentials: {
			username: "{username}",
			password: "{password}"
		}
	};
}
chrome.webRequest.onAuthRequired.addListener(callbackFn, {urls: ["<all_urls>"]}, ['blocking']);

manifest.json

{
  "version": "1.0.0",
  "manifest_version": 2,
  "name": "ChromeProxy",
  "permissions": [
    "proxy",
    "<all_urls>",
    "webRequest",
    "webRequestBlocking"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "minimum_chrome_version": "22.0.0"
}