Only enable mirroring for 2 displays

chrome/browser/resources/options/chromeos/display_options.js

Index: chrome/browser/resources/options/chromeos/display_options.js
diff --git a/chrome/browser/resources/options/chromeos/display_options.js b/chrome/browser/resources/options/chromeos/display_options.js
index ea6573cd9ae4ce6d3902912f8a82cb14bb32364e..cd544c881a4e241528006a0b37320f9d278cd96d 100644
--- a/chrome/browser/resources/options/chromeos/display_options.js
+++ b/chrome/browser/resources/options/chromeos/display_options.js
@@ -481,8 +481,16 @@ cr.define('options', function() {
           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;