Disable setting incognito prefs for extensions for now.

chrome/common/extensions/docs/samples.json

 {
   "api": {
     "chrome.bookmarks.create": "bookmarks.html#method-create",
     "chrome.bookmarks.export": "bookmarks.html#method-export",
     "chrome.bookmarks.get": "bookmarks.html#method-get",
     "chrome.bookmarks.getChildren": "bookmarks.html#method-getChildren",
     "chrome.bookmarks.getRecent": "bookmarks.html#method-getRecent",
     "chrome.bookmarks.getTree": "bookmarks.html#method-getTree",
     "chrome.bookmarks.import": "bookmarks.html#method-import",
     "chrome.bookmarks.move": "bookmarks.html#method-move",
@@ skipping 1543 matching lines @@
         "background.html",
         "manifest.json",
         "popup.html",
         "proxy_error_handler.js",
         "proxy_form_controller.js",
         "test\/jsunittest.js",
         "test\/proxy_form_controller_test.html",
         "test\/proxy_form_controller_test.js",
         "test\/unittest.css"
       ],
-      "source_hash": "7b3ff4599334eaa9f50aa70940bb5dead970665f",
+      "source_hash": "d4a52c15b1e5c8be5b26fe9305cf4aae0f2b0cc3",
       "zip_path": "examples\/extensions\/proxy_configuration.zip"
     },
     {
       "api_calls": [
         "chrome.browserAction.onClicked",
         "chrome.browserAction.setIcon",
         "chrome.extension.getBackgroundPage",
         "chrome.extension.getURL",
         "chrome.tabs.create",
         "chrome.tabs.get",
@@ skipping 248 matching lines @@
       "source_files": [
         "manifest.json",
         "popup.html",
         "sample.css"
       ],
       "source_hash": "c8bfc2b60f25c24dc3c5aaf75dd635c9419df66d",
       "zip_path": "examples\/api\/extension\/isAllowedAccess.zip"
     }
   ]
 }