Navi: A11y - Set aria-hidden in decorative image for Set Default page.

Bug: 937030
Change-Id: I44721aafb7e778f8887411b7c46e3d786bccd59d
Reviewed-on: https://chromium-review.googlesource.com/c/1496315Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636694}

chrome/browser/resources/welcome/onboarding_welcome/set_as_default/nux_set_as_default.html

@@ -77,7 +77,7 @@
       <div class="logo"></div>
       <h1 tabindex="-1">$i18n{setDefaultHeader}</h1>
       <h2>$i18n{setDefaultSubHeader}</h2>
-      <div class="illustration"></div>
+      <div class="illustration" aria-hidden="true"></div>
       <div class="button-bar">
         <paper-button id="decline-button" on-click="onDeclineClick_">
           $i18n{setDefaultSkip}