Minimal Webextension to reproduce chrome messaging issue on Chrome 71. Test site: http://jsfiddle.net/jqwidgets/6q7h9xcj/

background.js

chrome.runtime.onMessage.addListener((message, sender, respond) => {
  console.log('got message', message);
  respond({ action: 'reply' });
});

content-script.js

window.addEventListener('mousedown', () => {
  chrome.runtime.sendMessage({ action: 'test' }, reply => {
    console.log('got a reply');
  });
});

manifest.json

{
  "manifest_version": 2,
  "name": "Chrome message test",
  "version": "0.1",
  "content_scripts": [
    {
			"all_frames": true,
			"js": [
				"content-script.js"
			],
			"matches": [
				"http://*/*",
				"https://*/*"
			],
			"exclude_matches": [
				"*://*/_/chrome/newtab*"
			],
			"run_at": "document_start"
		}
  ],
  "permissions": [
    "http://*/*",
		"https://*/*"
  ],
  "background": {
		"scripts": [
			"background.js"
		]
	}
}