Only show start/stop mirroring button when there are multiple displays.

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.define('options', function() {
   var OptionsPage = options.OptionsPage;
 
   // The scale ratio of the display rectangle to its original size.
   /** @const */ var VISUAL_SCALE = 1 / 10;
 
@@ skipping 817 matching lines @@
     /**
      * Updates the description of the selected display section.
      * @private
      */
     updateSelectedDisplayDescription_: function() {
       if (this.focusedIndex_ == null ||
           this.displays_[this.focusedIndex_] == null) {
         $('selected-display-data-container').hidden = true;
         $('display-configuration-arrow').hidden = true;
         $('display-options-set-primary').hidden = true;
+        $('display-options-toggle-mirroring').hidden = true;
         return;
       }
 
       $('selected-display-data-container').hidden = false;
       var display = this.displays_[this.focusedIndex_];
       var nameElement = $('selected-display-name');
       while (nameElement.childNodes.length > 0)
         nameElement.removeChild(nameElement.firstChild);
       nameElement.appendChild(document.createTextNode(display.name));
 
@@ skipping 18 matching lines @@
       arrow.hidden = false;
       // Adding 1 px to the position to fit the border line and the border in
       // arrow precisely.
       arrow.style.top = $('display-configurations').offsetTop -
           arrow.offsetHeight / 2 + 1 + 'px';
       arrow.style.left = display.div.offsetLeft + display.div.offsetWidth / 2 -
           arrow.offsetWidth / 2 + 'px';
 
       $('display-options-set-primary').hidden =
           this.displays_[this.focusedIndex_].isPrimary;
+      $('display-options-toggle-mirroring').hidden =
+          (this.displays_.length <= 1);
     },
 
     /**
      * Clears the drawing area for display rectangles.
      * @private
      */
     resetDisplaysView_: function() {
       var displaysViewHost = $('display-options-displays-view-host');
       displaysViewHost.removeChild(displaysViewHost.firstChild);
       this.displaysView_ = document.createElement('div');
@@ skipping 201 matching lines @@
       mirroring, displays, layout, offset) {
     DisplayOptions.getInstance().onDisplayChanged_(
         mirroring, displays, layout, offset);
   };
 
   // Export
   return {
     DisplayOptions: DisplayOptions
   };
 });