Fix layout problem in the supervised user import dialog

Sorta-follow-up to https://codereview.chromium.org/911923002/

BUG=459121

Review URL: https://codereview.chromium.org/950873002

Cr-Commit-Position: refs/heads/master@{#317596}

chrome/browser/resources/options/supervised_user_import.css

@@ -34,6 +34,10 @@
   white-space: nowrap;
 }
 
+#supervised-user-list .already-on-this-device {
+  flex-shrink: 0;
+}
+
 #supervised-user-list > * {
   height: 40px;
 }