Settings: Fix closing the alert overlay opened from other overlays.

chrome/browser/resources/options2/alert_overlay.js

Index: chrome/browser/resources/options2/alert_overlay.js
diff --git a/chrome/browser/resources/options2/alert_overlay.js b/chrome/browser/resources/options2/alert_overlay.js
index a31201031284f89a70681e391ec628b2074e5db3..dbfd4098d3bf493f1395e96090ad830e86a61f6f 100644
--- a/chrome/browser/resources/options2/alert_overlay.js
+++ b/chrome/browser/resources/options2/alert_overlay.js
@@ -44,6 +44,14 @@ cr.define('options', function() {
       };
     },
 
+    /** @inheritDoc */
+    get nestingLevel() {
+      // AlertOverlay is special in that it is not tied to one page or overlay.
+      // Set the nesting level arbitrarily high so as to always be recognized as
+      // the top-most visible page.
+      return 99;
+    },
+
     /**
      * Handle the 'ok' button.  Clear the overlay and call the ok callback if
      * available.