Only show start/stop mirroring button when there are multiple 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 38afbbf75588f76e82561a0b21bcf88ae12ac795..25a9336dd5d2f41721e065427553d308b1e841ff 100644
--- a/chrome/browser/resources/options/chromeos/display_options.js
+++ b/chrome/browser/resources/options/chromeos/display_options.js
@@ -835,6 +835,7 @@ cr.define('options', function() {
         $('selected-display-data-container').hidden = true;
         $('display-configuration-arrow').hidden = true;
         $('display-options-set-primary').hidden = true;
+        $('display-options-toggle-mirroring').hidden = true;
         return;
       }
 
@@ -873,6 +874,8 @@ cr.define('options', function() {
 
       $('display-options-set-primary').hidden =
           this.displays_[this.focusedIndex_].isPrimary;
+      $('display-options-toggle-mirroring').hidden =
+          (this.displays_.length <= 1);
     },
 
     /**
@@ -1050,6 +1053,11 @@ cr.define('options', function() {
      * @private
      */
     onDisplayChanged_: function(mirroring, displays, layout, offset) {
+      if (displays.length <= 1) {
+        OptionsPage.showDefaultPage();

[hshi1, 2013/01/10 18:59:24]

My bad. This is causing all the chrome://settings/xxx pages to be re-directed to
the main settings page when only one monitor is attached. CL was already
reverted. Need to re-consider this.

[Jun Mukai, 2013/01/10 19:04:37]

Oops, I missed that point too :(  Sorry for my bad proposal.
Let's separate the patch, first create a CL of toggle-mirroring button, which is
apparently safe.

+        return;
+      }
+
       this.mirroring_ = mirroring;
       this.layout_ = layout;
       this.offset_ = offset;