demo mode: Vertically center illustrations on demo mode setup screen.

Change-Id: Ie36b88ba98d7190fab4bc613c7ad8714d7b1c118
Reviewed-on: https://chromium-review.googlesource.com/c/1282223Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600100}

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

@@ -34,7 +34,7 @@
       <h1 slot="title">
         [[i18nDynamic(locale, 'demoSetupProgressScreenTitle')]]
       </h1>
-      <div slot="footer" class="flex layout vertical center">
+      <div slot="footer" class="flex layout vertical center center-justified">
         <paper-spinner-lite id="spinner" dir="ltr" active></paper-spinner-lite>
       </div>
     </oobe-dialog>
@@ -48,7 +48,7 @@
       <div slot="subtitle">
         [[i18nDynamic(locale, 'demoSetupErrorScreenSubtitle')]]
       </div>
-      <div slot="footer" class="flex layout vertical center">
+      <div slot="footer" class="flex layout vertical center center-justified">
         <img srcset="images/alert-illustration_1x.svg 1x,
             images/alert-illustration_2x.svg 2x">
       </div>