Update the current mirroring mode correctly.

chrome/browser/resources/options/chromeos/display_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 {{
  *   availableColorProfiles: Array<{profileId: number, name: string}>,
  *   colorProfile: number,
@@ skipping 1002 matching lines @@
       this.mirroring_ = mirroring;
       this.unifiedDesktopEnabled_ = unifiedDesktopEnabled;
       this.displays_ = displays;
 
       this.resetDisplaysView_();
       if (this.mirroring_)
         this.layoutMirroringDisplays_();
       else
         this.layoutDisplays_();
 
+      $('display-options-select-mirroring').value =
+          mirroring ? 'mirroring' : 'extended';
+
       $('display-options-unified-desktop').hidden =
           !this.showUnifiedDesktopOption_;
 
       $('display-options-toggle-unified-desktop').checked =
           this.unifiedDesktopEnabled_;
 
       var disableUnifiedDesktopOption =
            (this.mirroring_ ||
             (!this.unifiedDesktopEnabled_ &&
               this.displays_.length == 1));
 
       $('display-options-toggle-unified-desktop').disabled =
           disableUnifiedDesktopOption;
 
       this.updateSelectedDisplayDescription_();
     }
   };
 
   DisplayOptions.setDisplayInfo = function(
       mode, displays, layout, offset) {
     DisplayOptions.getInstance().onDisplayChanged_(
         mode, displays, layout, offset);
   };
 
   // Export
   return {
     DisplayOptions: DisplayOptions
   };
 });