Multidevice setup: Set overflow to visible in the session.

This eliminates unnecessary scrollbar in the 'Enter Your password' screen

Bug: 1022335
Change-Id: Ib2254d58a1a91c4cd0a198385aa85ba7bf1e0407
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906787
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714187}

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

@@ -32,6 +32,7 @@
       multidevice-setup {
         --multidevice-setup-height: 100%;
         --multidevice-setup-width: 100%;
+        --iron-pages-overflow: auto;
       }
 
       #multidevice-help-overlay-container {

chrome/browser/resources/chromeos/multidevice_setup/multidevice_setup_post_oobe.html

@@ -13,6 +13,7 @@
       multidevice-setup {
         --multidevice-setup-height: 640px;
         --multidevice-setup-width: 768px;
+        --iron-pages-overflow: visible;
       }
 
       #backward-button,

ui/webui/resources/cr_components/chromeos/multidevice_setup/multidevice_setup.html

@@ -28,7 +28,7 @@
       }
 
       iron-pages {
-        overflow: auto;
+        overflow: var(--iron-pages-overflow, visible);
         padding: 0 32px;
       }
     </style>