Only enable mirroring for 2 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.exportPath('options');
 
 /**
  * Enumeration of multi display mode. These values must match the C++ values in
  * ash::DisplayManager.
  * @enum {number}
@@ skipping 463 matching lines @@
       var arrow = $('display-configuration-arrow');
       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 + 'px';
       arrow.style.left = displayLayout.div.offsetLeft +
           displayLayout.div.offsetWidth / 2 - arrow.offsetWidth / 2 + 'px';
 
       $('display-options-set-primary').disabled = display.isPrimary;
-      $('display-options-select-mirroring').disabled =
-          (this.displays_.length <= 1 && !this.unifiedDesktopEnabled_);
+
+      // Mirroring is only supported for 2 displays.
+      var allowMirroring = this.displays_.length == 2;
+      if (this.unifiedDesktopEnabled_) {
+        // In unified desktop mode, there is only one display as far as the
+        // UI is concerned, but we want to allow the user to select mirroring
+        // (which will turn off unified mode).
+        allowMirroring = true;

[oshima, 2016/03/30 18:51:22]

The original code was based on the assumption that there are at most 2 displays
connected.

It's proobably better to pass the boolean flag if mirroring is enabled rather
than having logic here. (DisplayManager::num_connected_displays() == 2)

[stevenjb, 2016/03/30 19:03:30]

I don't understand your suggestion. Do you want me to move this logic to C++?
I'm not sure what the advantage to that would be.

+      }
+      $('display-options-select-mirroring').disabled = !allowMirroring;
       $('selected-display-start-calibrating-overscan').disabled =
           display.isInternal;
 
       var orientation = $('display-options-orientation-selection');
       orientation.disabled = this.unifiedDesktopEnabled_;
 
       var orientationOptions = orientation.getElementsByTagName('option');
       var orientationIndex = Math.floor(display.rotation / 90);
       orientationOptions[orientationIndex].selected = true;
 
@@ skipping 217 matching lines @@
 
   DisplayOptions.setDisplayInfo = function(mode, displays) {
     DisplayOptions.getInstance().onDisplayChanged_(mode, displays);
   };
 
   // Export
   return {
     DisplayOptions: DisplayOptions
   };
 });