Disable setting incognito prefs for extensions for now.

chrome/test/data/extensions/api_test/content_settings/incognito/test.html

 <script>
 // Content settings API test
 // Run with browser_tests --gtest_filter=ExtensionApiTest.IncognitoContentSettings
 
 var cs = chrome.experimental.contentSettings;
 function expect(expected, message) {
   return chrome.test.callbackPass(function(value) {
     chrome.test.assertNoLastError();
     chrome.test.assertEq(expected, value, message);
   });
 }
 chrome.test.runTests([
   function getRegular() {
     cs.misc.blockThirdPartyCookies.get(
         {},
         expect({ 'value': false,
                  'levelOfControl': "ControllableByThisExtension" },
                "third-party cookies should not be blocked"));
   },
   function getIncognito() {
     cs.misc.blockThirdPartyCookies.get(
         { 'incognito': true },
         expect({ 'value': false,
                  'incognitoSpecific': false,
                  'levelOfControl': "ControllableByThisExtension" },
                "third-party cookies should not be blocked in incognito mode"));
   },
+  // TODO(battre): re-enable when incognito preferences are available again.
+  /*
   function set() {
     cs.misc.blockThirdPartyCookies.set(
         { 'incognito': true, 'value': true }, chrome.test.callbackPass());
   },
   function getRegular2() {
     cs.misc.blockThirdPartyCookies.get(
         {},
         expect({ 'value': false,
                  'levelOfControl': "ControllableByThisExtension" },
                "third-party cookies should not be blocked"));
   },
   function getIncognito2() {
     cs.misc.blockThirdPartyCookies.get(
         { 'incognito': true },
         expect({ 'value': true,
                  'incognitoSpecific': true,
                  'levelOfControl': "ControlledByThisExtension" },
                "third-party cookies should be blocked in incognito mode"));
   },
+  */
 ]);
 </script>