Chrome extension with declarativeContent.RequestContentScript

I am trying to make a Chrome extension, which will monitor GMail and do something when user starts to write a message. After some study of examples and documentation I have figured out that I should do it with declarativeContent, which reacts on page change.

This is what I have done by now.

manifest.json:

{
  "manifest_version": 2,
  "name": "Gmail helper",
  "version": "0.1",
  "permissions": [ "declarativeContent" ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  }
}

background.js:

chrome.runtime.onInstalled.addListener (function (details) {
  chrome.declarativeContent.onPageChanged.removeRules (undefined, function () {
    chrome.declarativeContent.onPageChanged.addRules ([{
      conditions: [
        new chrome.declarativeContent.PageStateMatcher({
          pageUrl: { hostEquals: 'mail.google.com', schemes: ['https'] },
          css: ["div"]
//          css: ["div[aria-label='Message Body']"]
        })
      ],
      actions: [ new chrome.declarativeContent.RequestContentScript({js: ["content.js"]}) ]
    }]);
  });
});

content.js:

alert ("Test");

My plan was to declare a content script that would trigger on page changes in GMail. I have added a declarative rule, which has pageURL, css and actions defined. According to my understanding content.js should be executed when pageUrl and css content match. However, content.js is not executed.

What am I doing wrong?

Thanks.