Fix Regression failure on Multiprofile screen.

In case of multipofile mode, when account picker is invoked a blank
white screen appears. Though it is not entirely clear why it happens,
but removing 'full-screen-dialog' attribute from the oobe-dialogs in
checking_downloading_update polymer element apparently fixes it. This
leads to another problem of the update_screen not shown in full screen
mode. For that we need to call onBeforeShow() of the polymer element
explicitly so that the oobe_dialog_host_behavior in the
checking_downlaoding_update can be used to set full screen mode for
all the oobe dialogs.

Bug: 1061330
Change-Id: Ic51082b0694f4bed0f357ec7616ab2fbc108fcb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2105316Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Cr-Commit-Position: refs/heads/master@{#750905}

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

@@ -44,8 +44,7 @@
     <link rel="stylesheet" href="checking_downloading_update.css">
     <oobe-dialog hidden="[[!checkingForUpdate]]"
         title-key="checkingForUpdates"
-        aria-live="polite" id="checking-for-updates-dialog" footer-shrinkable
-        full-screen-dialog>
+        aria-live="polite" id="checking-for-updates-dialog" footer-shrinkable>
       <hd-iron-icon slot="oobe-icon"
           icon1x="oobe-32:googleg" icon2x="oobe-64:googleg">
       </hd-iron-icon>
@@ -70,7 +69,7 @@
         title-key="updatingScreenTitle"
         hidden="[[isCheckingOrUpdateCompleted_(checkingForUpdate,
                                                updateCompleted)]]"
-        aria-live="polite" full-screen-dialog>
+        aria-live="polite">
       <hd-iron-icon slot="oobe-icon"
           icon1x="oobe-32:googleg" icon2x="oobe-64:googleg">
       </hd-iron-icon>
@@ -97,7 +96,7 @@
 
     <oobe-dialog footer-shrinkable id="update-complete-dialog"
         hidden="[[!updateCompleted]]" subtitle-key="updateCompeletedMsg"
-        aria-live="polite" full-screen-dialog>
+        aria-live="polite">
       <hd-iron-icon slot="oobe-icon"
           icon1x="oobe-32:googleg" icon2x="oobe-64:googleg">
       </hd-iron-icon>

chrome/browser/resources/chromeos/login/oobe_update.js

@@ -90,6 +90,7 @@ Polymer({
       if (behavior.onBeforeShow)
         behavior.onBeforeShow.call(this);
     });
+    this.$['checking-downloading-update'].onBeforeShow();
   },
 
   onBackClicked_() {