crostini: Add a missing override annotation.

Bug: None
Test: CQ
Change-Id: I109e5ca8ad670160a71d3296c2f9f32ceaff14c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065509
Commit-Queue: David Munro <davidmunro@google.com>
Commit-Queue: calamity <calamity@chromium.org>
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742952}

chrome/browser/resources/settings/chromeos/crostini_page/crostini_browser_proxy.js

@@ -190,6 +190,7 @@ cr.define('settings', function() {
       chrome.send('requestCrostiniUpgraderDialogStatus');
     }
 
+    /** @override */
     addCrostiniPortForward(
         vmName, containerName, portNumber, protocolIndex, label) {
       return cr.sendWithPromise(