cros: Update migration banner on login

ui/login/account_picker/screen_account_picker.css

 /* Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 .account-picker.flying-pods #account-picker {
   transition: width 180ms ease, height 180ms ease;
 }
 
 #bubble {
   z-index: 1;
 }
 
 #signin-banner-container1 {
   bottom: 100%;
   margin-bottom: 10px;
   position: absolute;
   width: 100%;
 }
 
+html[screen=login] #signin-banner-container1 {
+  margin-bottom: 6px;
+}
+
 #signin-banner-container2 {
   display: inline-block;
   position: relative;
   right: -50%;
 }
 
 html[dir=rtl] #signin-banner-container2 {
   left: -50%;
   right: auto;
 }
 
 #signin-banner {
   background-color: rgba(0, 0, 0, 0.75);
   border-radius: 4px;
   color: whitesmoke;
   display: none;
-  font-size: 15px;
   left: -50%;
+  max-width: 722px;
   padding: 20px;
   position: relative;
   text-align: center;
-  width: 722px;
+  width: max-content;
 }
 
 html[dir=rtl] #signin-banner {
   left: auto;
   right: -50%;
 }
 
 html[screen=user-adding] #signin-banner {
   display: inline-block;
 }
 
+html[screen=login] #signin-banner {
+  padding: 20px 24px;
+}
+
 html[screen=login] #signin-banner,
 html[screen=lock] #signin-banner {
   display: inline-block;
   opacity: 0;
   visibility: hidden;
 }
 
 html[screen=login] #signin-banner.message-set,
 html[screen=lock] #signin-banner.message-set {
   opacity: 1;
   transition: opacity 1s;
   visibility: visible;
 }