crostini: Add a missing break to stop accidental switch fallthrough.

Bug: None
Test: CQ
Change-Id: I0b1ef13b013673712abfe69f25fe2ef1dc22d3fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000438
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#731850}

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

@@ -360,11 +360,13 @@ Polymer({
           default:
             assertNotReached();
         }
+        break;
       case State.UPGRADING:
         messageId = 'upgradingMessage';
         break;
       case State.SUCCEEDED:
         messageId = 'succeededMessage';
+        break;
     }
     return messageId ? loadTimeData.getString(messageId) : '';
   },