title: Disable chrome "Add extension" prompt
body: <p>I am automating the testing process of installing a plugin in selenium right now. I am well aware you can include chrome plugins in the <code>chromeOptions</code> but what I want to test here is the process of manually installing the extension and how my web app reacts to it.</p>
  
  <p>After clicking the "Add to Chrome" button in the web store, a prompt appears and you have to click "Add extension". I already know it is not possible to confirm this prompt using selenium, so my idea is to allow installing extensions without any prompts.
  According to <a href="https://peter.sh/experiments/chromium-command-line-switches/#prompt-for-external-extensions" rel="nofollow noreferrer">this</a> I can use the <code>--prompt-for-external-extensions</code> flag to disable those prompts.</p>
  
  <p>All my attempts to add this flag to the <code>ChromeOptions</code> so far have failed and the prompt still appears. I am also unsure whether the precondition <code>The constant CHROMIUM_BUILD must be defined.</code> is fulfilled.</p>
  
  <p>My attempts:</p>
  
  <pre><code>ChromeOptions options = new ChromeOptions();
  options.addArguments("--prompt-for-external-extensions");
  //or this
  options.setExperimentalOption("excludeSwitches", Arrays.asList("prompt-for-external-extensions"));
  </code></pre>
  
  <p>How can I succesfully disable this prompt in chrome?</p>