cros: Selectively fork login assets.

ui/login/account_picker/md_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;
+  left: -50%;
+  max-width: 722px;
+  padding: 20px;
+  position: relative;
+  text-align: center;
+  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;
+}