Make oobe dialogs container positioned

Needed so nested content correctly positioned against the containers

Bug: 1000606
Change-Id: I6cae1e149e546081994894e4bb91021f52b47873
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864772Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706819}

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

@@ -90,3 +90,7 @@
   --iron-icon-width: 32px;
   --iron-icon-fill-color: var(--google-blue-600);
 }
+
+.slot-container {
+  position: relative;
+}

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

@@ -63,26 +63,26 @@
     <link rel="stylesheet" href="oobe_flex_layout.css">
     <style include="cr-shared-style"></style>
     <div id="header-container" hidden="[[noHeader]]">
-      <div id="oobe-icon-div">
+      <div id="oobe-icon-div" class="slot-container">
         <slot name="oobe-icon"></slot>
       </div>
-      <div id="oobe-title" class="layout vertical end-justified">
+      <div id="oobe-title" class="slot-container layout vertical end-justified">
         <slot name="title"></slot>
       </div>
-      <div id="oobe-subtitle" class="layout vertical">
+      <div id="oobe-subtitle" class="slot-container layout vertical">
         <slot name="subtitle"></slot>
       </div>
-      <div id="oobe-progress" class="layout vertical">
+      <div id="oobe-progress" class="slot-container layout vertical">
         <slot name="progress"></slot>
       </div>
     </div>
     <div id="footer-container" noFooterPadding$="[[noFooterPadding]]"
-        class="flex layout vertical">
+        class="slot-container flex layout vertical">
       <slot name="footer"></slot>
     </div>
     <template is="dom-if" if="[[hasButtons]]">
       <div id="oobe-bottom" hideShadow$="[[hideShadow]]"
-          class="layout horizontal center">
+          class="slot-container layout horizontal center">
         <slot name="bottom-buttons"></slot>
       </div>
     </template>