settings: Prevent hitting enter on setup PIN dialog to reopen it.

Using the approach suggested in [1].

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/1100235

Test: manual
Bug: 845686
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I66a8cb687463c30ba2414c29245797338a49d60f
Reviewed-on: https://chromium-review.googlesource.com/1103280
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568190}

chrome/browser/resources/settings/people_page/lock_screen.html

@@ -110,8 +110,12 @@
                   <div class="separator"></div>
                   <div id="pinPasswordSecondaryActionDiv"
                       class="secondary-action">
+                    <!-- Use stop-keyboard-event-propagation to prevent
+                         triggering this when focused after closing the
+                         dialog. -->
                     <paper-button id="setupPinButton" class="secondary-button"
-                        on-click="onConfigurePin_">
+                        on-click="onConfigurePin_"
+                        stop-keyboard-event-propagation>
                       [[getSetupPinText_(hasPin)]]
                     </paper-button>
                   </div>