cros: Dialogs should always be full-screen during OOBE

The dialogs should always be full-screen as long as there's no user
added yet. This covers the corner cases that user stops in the middle
of OOBE and returns later.

Do not show the dialog as full-screen (at least for now, until there's
UX update) for cases that the OOBE is completed, but user pods are not
shown (due to policy, or all users are removed manually on an enrolled
device).

Bug: 872936
Change-Id: I02a648585a7cbebd1b570096ab3608bff000e96d
Reviewed-on: https://chromium-review.googlesource.com/1171916Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582437}

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

@@ -65,7 +65,9 @@ Polymer({
   },
 
   onBeforeShow: function() {
-    if (document.documentElement.getAttribute('full-screen-dialog'))
+    var isOobe = Oobe && Oobe.getInstance() &&
+        Oobe.getInstance().displayType == DISPLAY_TYPE.OOBE;
+    if (isOobe || document.documentElement.getAttribute('full-screen-dialog'))
       this.fullScreenDialog = true;
   },