[ChromeOS] Show a loading UI before Gaia is loaded for WebUI login.

chrome/browser/resources/chromeos/login/screen_signin.css

Index: chrome/browser/resources/chromeos/login/screen_signin.css
diff --git a/chrome/browser/resources/chromeos/login/screen_signin.css b/chrome/browser/resources/chromeos/login/screen_signin.css
index 5d55d8b74bd0b0e78c81c8897282fa340dd6bc6d..6f5fc16cf10e4f47630918b7921f70618f3338a9 100644
--- a/chrome/browser/resources/chromeos/login/screen_signin.css
+++ b/chrome/browser/resources/chromeos/login/screen_signin.css
@@ -27,9 +27,8 @@
   margin-top: 20px;
 }
 
-#gaia-signin > iframe {
+#signin-frame {
   bottom: 0;
-  display: -webkit-box;
   height: 100%;
   left: 0;
   position: absolute;
@@ -38,6 +37,17 @@
   width: 340px;
 }
 
+#gaia-loading {
+  -webkit-box-align: center;
+  -webkit-box-pack: center;
+  bottom: 0;
+  display: -webkit-box;
+  left: 0;
+  position: absolute;
+  right: 0;
+  top: 0;
+}
+
 #email,
 #password {
   display: block;