Prevent overlay enter events from executing two buttons.

ui/webui/resources/js/cr/ui/overlay.js

Index: ui/webui/resources/js/cr/ui/overlay.js
diff --git a/ui/webui/resources/js/cr/ui/overlay.js b/ui/webui/resources/js/cr/ui/overlay.js
index 293ca20f2caa3175bd89deeff09b3748e3b05451..5c0acb2d3dd826d52e3f76e1a3de7b7127671043 100644
--- a/ui/webui/resources/js/cr/ui/overlay.js
+++ b/ui/webui/resources/js/cr/ui/overlay.js
@@ -58,8 +58,13 @@ cr.define('cr.ui.overlay', function() {
         if (e.keyIdentifier == 'Enter' &&
             !forbiddenTagNames.test(document.activeElement.tagName)) {
           var button = getDefaultButton(overlay);
-          if (button)
+          if (button) {
             button.click();
+            // Executing the default button may result in focus moving to a
+            // different button. Calling preventDefault is necessary to not have
+            // that button execute as well.
+            e.preventDefault();
+          }
         }
       });