[Kiosk] Move network diagnostic buttons to the footer

In some cases, when the number of networks was too big, the network
configuration buttons were not visible. Let's move them to the footer
so they will always be there.

Bug: 1130381
Change-Id: I67cbcab2b014a4662a93a9472c1a40e0e8525215
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537355Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Anatoliy Potapchuk <apotapchuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827827}

chrome/browser/resources/chromeos/login/screen_error_message.html

@@ -134,8 +134,12 @@
           </network-select-login>
         </div>
       </div>
-      <div id="error-message-md-footer-spacer" class="flex"></div>
-      <div id="buttons" class="layout horizontal center end-justified">
+    </div>
+    <div slot="back-navigation">
+      <oobe-back-button id="error-message-back-button"></oobe-back-button>
+    </div>
+    <div slot="bottom-buttons">
+      <div id="buttons" class="layout horizontal">
         <oobe-text-button id="error-message-md-reboot-button"
             text-key="rebootButton"
             class="show-with-ui-state-kiosk-mode">
@@ -162,10 +166,6 @@
             class="show-with-ui-state-local-state-error">
         </oobe-text-button>
       </div>
-      <div id="error-message-md-bottom-margin"></div>
-    </div>
-    <div slot="back-navigation">
-      <oobe-back-button id="error-message-back-button"></oobe-back-button>
     </div>
   </oobe-dialog>
 </div>