Allow extension APIs to be called from WebUI.

chrome/test/data/extensions/webui/on_message.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.OnMessage
+
+chrome.test.listenOnce(chrome.test.onMessage, function(args) {
+  if (args.data != 'hi') {
+    console.error('Expected "hi", Actual ' + JSON.stringify(args.data));
+    chrome.test.sendMessage('false');
+  } else {
+    chrome.test.sendMessage('true');
+  }
+});
+
+domAutomationController.send(true);