Crostin upgrade: re-enable cancel button

Bug: 1103505
Change-Id: I162fa82ecd13a43117ce6be1e098591bb3518674
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2320710
Commit-Queue: David Munro <davidmunro@google.com>
Reviewed-by: David Munro <davidmunro@google.com>
Auto-Submit: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792501}

chrome/browser/resources/chromeos/crostini_upgrader/app.js

@@ -341,7 +341,6 @@ Polymer({
    */
   canCancel_(state) {
     switch (state) {
-      case State.UPGRADING:  // TODO(nverne): remove once we have OK from UX.
       case State.BACKUP:
       case State.RESTORE:
       case State.BACKUP_SUCCEEDED: