Center lock screen user pod when virtual keyboard is shown.

chrome/browser/resources/login/screen_container.css

Index: chrome/browser/resources/login/screen_container.css
diff --git a/chrome/browser/resources/login/screen_container.css b/chrome/browser/resources/login/screen_container.css
index 6a1d6afd66d2b204fd19d7ffa9da8fc85e3b099a..475a26789da767ada45c6af3694ac1ec23f35a1d 100644
--- a/chrome/browser/resources/login/screen_container.css
+++ b/chrome/browser/resources/login/screen_container.css
@@ -10,12 +10,15 @@
   bottom: 51px;  /* Leave space for the header bar */
   display: -webkit-box;
   left: 0;
-  min-height: 717px; /* This enables scrolling. Min resolution: 1024x768 */
   position: absolute;
   right: 0;
   top: 0;
 }
 
+html[not(screen=lock)] #outer-container {
+  min-height: 717px; /* This enables scrolling. Min resolution: 1024x768 */
+}
+
 .oobe-display #outer-container.down {
   -webkit-perspective: 600px;
 }