Chrome OS First run: add role="dialog" to the steps

Bug: 890464
Change-Id: I8fe561d676089b2cb873b846314042e109913a64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872223Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708491}

chrome/browser/resources/chromeos/first_run/app_list_step.html

-<div id="app-list" class="step bubble transparent hidden">
-  <h1 i18n-content="appListHeader"></h1>
+<div id="app-list" role="dialog" aria-labelledby="appListHeader"
+    class="step bubble transparent hidden">
+  <h1 id="appListHeader" i18n-content="appListHeader"></h1>
   <p>
     <span i18n-content="appListText1"></span><br>
     <span id="google-assistant-text" i18n-content="appListText2" hidden></span>

chrome/browser/resources/chromeos/first_run/tray_step.html

-<div id="tray" class="step bubble transparent hidden">
-  <h1 i18n-content="trayHeader"></h1>
+<div id="tray" role="dialog" aria-labelledby="trayHeader"
+    class="step bubble transparent hidden">
+  <h1 id="trayHeader" i18n-content="trayHeader"></h1>
   <p i18n-content="trayText"><p>
   <div class="controls">
     <button i18n-content="helpKeepExploringButton"