Center the focus of the text input in magnifier screen.

chrome/browser/resources/options/browser_options.js

Index: chrome/browser/resources/options/browser_options.js
diff --git a/chrome/browser/resources/options/browser_options.js b/chrome/browser/resources/options/browser_options.js
index d9456b7f888e7cc85725829b41027b58087107a0..8760cd1256a2bc603c3838ca789651b78fda6a7d 100644
--- a/chrome/browser/resources/options/browser_options.js
+++ b/chrome/browser/resources/options/browser_options.js
@@ -682,6 +682,14 @@ cr.define('options', function() {
             updateAccessibilitySettingsButton;
         updateAccessibilitySettingsButton();
 
+        var updateScreenMagnifierCenterFocus = function() {
+          $('accessibility-screen-magnifier-center-focus-check').disabled =
+              !$('accessibility-screen-magnifier-check').checked;
+        };
+        Preferences.getInstance().addEventListener(
+            $('accessibility-screen-magnifier-check').getAttribute('pref'),
+            updateScreenMagnifierCenterFocus);
+
         var updateDelayDropdown = function() {
           $('accessibility-autoclick-dropdown').disabled =
               !$('accessibility-autoclick-check').checked;