Implement chrome.systemPrivate.getIncognitoModeAvailability extension API function

chrome/test/data/extensions/api_test/system/test.html

+<script>
+// System API test
+// Run with browser_tests --gtest_filter=ExtensionApiTest.SystemApi
+
+function expect(expected, message) {
+  return chrome.test.callbackPass(function(value) {
+    chrome.test.assertEq(expected, value, message);
+  });
+}
+
+chrome.test.runTests([
+  function readOnlyGetTest() {
+    chrome.systemPrivate.incognitoModeAvailability.get({}, expect(
+        {value: 'disabled', levelOfControl: 'not_controllable'},
+        "Pref `chrome.windows.incognitoModeAvailability` is expected to be " +
+        "'disabled'"));
+  },
+  function readOnlySetTest() {
+    chrome.systemPrivate.incognitoModeAvailability.set(
+        {value: 'enabled'},
+        chrome.test.callbackFail("You cannot modify the read-only preference " +
+                                 "'incognitoModeAvailability'."));
+  }
+]);
+</script>