Add a very basic PIN UI implementation that is shared between lock and settings.

ui/login/screen_container.css

Index: ui/login/screen_container.css
diff --git a/ui/login/screen_container.css b/ui/login/screen_container.css
index 57742eaf6784d494d73f4707ee923bf706e6aaf3..9a9177503d6ef95adec7a980da4fb9146826776d 100644
--- a/ui/login/screen_container.css
+++ b/ui/login/screen_container.css
@@ -21,6 +21,11 @@
   -webkit-perspective: 600px;
 }
 
+#pin-container {
+  opacity: 0; /* Opacity is set to 1 after the pin element is loaded. */
+  transition: opacity 200ms ease-in-out;
+}
+
 #scroll-container {
   bottom: 0;        /* Allows content overlap with control bar. */
   left: 0;