Center the focus of the text input in magnifier screen.

chrome/browser/resources/options/browser_options.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.exportPath('options');
 
 /**
  * @typedef {{actionLinkText: (string|undefined),
  *            childUser: (boolean|undefined),
  *            hasError: (boolean|undefined),
@@ skipping 664 matching lines @@
           chrome.send('coreOptionsUserMetricsAction',
                       ['Options_AccessibilityLearnMore']);
         };
         $('accessibility-settings-button').onclick = function(unused_event) {
           window.open(loadTimeData.getString('accessibilitySettingsURL'));
         };
         $('accessibility-spoken-feedback-check').onchange =
             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;
         };
         Preferences.getInstance().addEventListener(
             $('accessibility-autoclick-check').getAttribute('pref'),
             updateDelayDropdown);
       }
 
       // Display management section (CrOS only).
@@ skipping 1574 matching lines @@
       }
       button.textContent = loadTimeData.getString(strId);
     };
   }
 
   // Export
   return {
     BrowserOptions: BrowserOptions
   };
 });