Allow extension APIs to be called from WebUI.

chrome/test/data/extensions/webui/runtime_last_error.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// out/Debug/browser_tests --gtest_filter=ExtensionWebUITest.RuntimeLastError
+
+var success = true;
+
+if (!chrome.test.checkDeepEq({}, chrome.runtime)) {
+  console.error('Expected {}, Actual ' + JSON.stringify(chrome.runtime));
+  success = false;
+}
+
+chrome.test.sendMessage('ping', function(reply) {
+  var expected = {
+    'lastError': {
+      'message': 'unknown host'
+    }
+  };
+  if (!chrome.test.checkDeepEq(expected, chrome.runtime)) {
+    console.error('Expected ' + JSON.stringify(expected) + ', ' +
+                  'Actual ' + JSON.stringify(chrome.runtime));
+    success = false;
+  }
+  chrome.test.sendMessage(success ? 'true' : 'false');
+});
+
+domAutomationController.send(true);