OOBE: Fix default focus for Enterprise enrollment screen

Focusing element marked as "focus-on-show" is done by oobe-dialog's
show() method. MultiStepBehavior triggers this method for elements
marked as for-step, but enrollment_screen used legacy layout where
each oobe-dialog was additionally wrapped in the <div>, and for-step
was set on the enclosing div instead of oobe-dialog.
As a result, show() was not called on corresponding oobe-dialog upon
step switch, and default focus was not set

Bug: 1021608
Change-Id: I5d103f735ff5155a190c0a74c7d215fe293a2c83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414189Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807869}

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

@@ -41,8 +41,7 @@
       </div>
 
       <!-- ENROLLMENT IN PROGRESS (SPINNER) DIALOG -->
-      <div id="step-working" for-step="working">
-          <oobe-dialog id="working"
+      <oobe-dialog id="step-working" for-step="working"
           title-key="oauthEnrollScreenTitle">
         <hd-iron-icon slot="oobe-icon"
             icon1x="oobe-32:enterprise" icon2x="oobe-64:enterprise">
@@ -55,7 +54,6 @@
           </div>
         </div>
       </oobe-dialog>
-      </div>
       <div id="step-ad-join" for-step="ad-join">
         <offline-ad-login id="adJoinUI" is-domain-join
             class="fit"
@@ -74,9 +72,8 @@
               AttributePrompt: [ 'Done' ]
               ActiveDirectoryJoin: [ 'Try Again' ]
       -->
-      <div id="step-error" role="alert"
+      <oobe-dialog id="step-error" has-buttons role="alert"
           for-step="error, attribute-prompt-error, active-directory-join-error">
-        <oobe-dialog id="errorCard" has-buttons>
         <hd-iron-icon slot="oobe-icon"
                       icon1x="oobe-32:enterprise"
                       icon2x="oobe-64:enterprise"></hd-iron-icon>
@@ -133,11 +130,9 @@
 
         </div>
       </oobe-dialog>
-      </div>
 
       <!-- SUCCESS DIALOG -->
-      <div id="step-success" for-step="success" role="alert">
-        <oobe-dialog id="successCard" has-buttons
+      <oobe-dialog has-buttons id="step-success" for-step="success" role="alert"
           title-key="oauthEnrollSuccessTitle" footer-shrinkable>
         <hd-iron-icon slot="oobe-icon"
             icon1x="oobe-32:enterprise" icon2x="oobe-64:enterprise">
@@ -160,12 +155,10 @@
               class="focus-on-show"></oobe-text-button>
         </div>
       </oobe-dialog>
-      </div>
 
       <!-- ATTRIBUTE PROMPT DIALOG -->
-      <div id="step-attribute-prompt" for-step="attribute-prompt">
-        <oobe-dialog id="attributePromptCard" has-buttons
-            title-key="oauthEnrollScreenTitle"
+      <oobe-dialog id="step-attribute-prompt" for-step="attribute-prompt"
+          has-buttons title-key="oauthEnrollScreenTitle"
           subtitle-key="oauthEnrollDeviceInformation">
         <hd-iron-icon slot="oobe-icon"
             icon1x="oobe-32:enterprise" icon2x="oobe-64:enterprise">
@@ -200,7 +193,6 @@
         </div>
       </oobe-dialog>
     </div>
-    </div>
     <div class="popup-overlay"
         hidden="[[!showPopupOverlay_(authenticatorDialogDisplayed_,
                                      isSamlSsoVisible_)]]">