ChromeOS OOBE: make "System Security Settings" popup dialog backgroung gray.

This CL makes background style of "System Security Settings" look like
"Help App" background dialog that can be launched from the same UI.

BUG=604119,665752
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2648623002
Cr-Commit-Position: refs/heads/master@{#445024}

chrome/browser/resources/chromeos/login/oobe.js

@@ -333,9 +333,11 @@ cr.define('cr.ui.Oobe', function() {
       if (loadTimeData.getString('newOobeUI') == 'on') {
         $('oobe').setAttribute('md-mode', 'true');
         $('oobe-shield').setAttribute('md-mode', 'true');
+        $('popup-overlay').setAttribute('md-mode', 'true');
       } else {
         $('oobe').removeAttribute('md-mode');
         $('oobe-shield').removeAttribute('md-mode');
+        $('popup-overlay').removeAttribute('md-mode');
       }
     },
   };

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

@@ -17,6 +17,10 @@
   z-index: 5;
 }
 
+#popup-overlay[md-mode] {
+  background: rgba(0, 0, 0, .5);
+}
+
 .oobe-popup {
   background: white;
   border: 1px solid rgb(188, 193, 208);