Implement chrome.systemPrivate.getIncognitoModeAvailability extension API function

chrome/test/data/extensions/api_test/preference/standard/test.html

 <script>
 // Preferences API test
 // Run with browser_tests --gtest_filter=ExtensionApiTest.PreferenceApi
 
 var preferences_to_test = [
   {
     root: chrome.experimental.privacy.network,
     preferences: [
       'networkPredictionEnabled'
     ]
@@ skipping 46 matching lines @@
     for (var i = 0; i < preferences_to_test.length; i++) {
       preferences_to_test[i].preferences.forEach(
           prefGetter.bind(preferences_to_test[i].root));
     }
   },
   function setGlobals() {
     for (var i = 0; i < preferences_to_test.length; i++) {
       preferences_to_test[i].preferences.forEach(
           prefSetter.bind(preferences_to_test[i].root));
     }
+  },
+  function readOnlyGetTest() {
+    chrome.windows.incognitoModeAvailability.get({}, expect(
+        {value: 'disabled', levelOfControl: 'not_controllable'},
+        "Pref `chrome.windows.incognitoModeAvailability` is expected to be " +
+        "'disabled'"));
+  },
+  function readOnlySetTest() {
+    chrome.windows.incognitoModeAvailability.set(
+        {value: 'enabled'},
+        chrome.test.callbackFail("You cannot modify the read-only preference " +
+                                 "'incognitoModeAvailability'."));
   }
 ]);
 </script>