Implement chrome.systemPrivate.getIncognitoModeAvailability extension API function

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

Index: chrome/test/data/extensions/api_test/preference/standard/test.html
diff --git a/chrome/test/data/extensions/api_test/preference/standard/test.html b/chrome/test/data/extensions/api_test/preference/standard/test.html
index 95864413066dde37f32b04cda9e596e4231bed93..9081b2a6a0dddd7f32d66c9f39f4c466b84cccde 100644
--- a/chrome/test/data/extensions/api_test/preference/standard/test.html
+++ b/chrome/test/data/extensions/api_test/preference/standard/test.html
@@ -64,6 +64,18 @@ chrome.test.runTests([
       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>