Disable setting incognito prefs for extensions for now.

chrome/renderer/resources/extension_process_bindings.js

Index: chrome/renderer/resources/extension_process_bindings.js
diff --git a/chrome/renderer/resources/extension_process_bindings.js b/chrome/renderer/resources/extension_process_bindings.js
index 0c4c9ff76c922c6ed765878ed3d4f8f457961232..89fd3d44e16335378fed02038950a91ce4ef5147 100644
--- a/chrome/renderer/resources/extension_process_bindings.js
+++ b/chrome/renderer/resources/extension_process_bindings.js
@@ -370,8 +370,8 @@ var chrome = chrome || {};
                            [prefKey, details, callback],
                            extendSchema(clearSchema));
       };
-      this.onChange = new chrome.Event('experimental.preferences.' + prefKey
-                                           + '.onChange');
+      this.onChange = new chrome.Event('experimental.preferences.'
+          + prefKey + '.onChange');
     };
     customBindings['Preference'].prototype = new CustomBindingsObject();
   }