Add a mojoPrivate API that exposes mojo to component apps/extensions.

extensions/renderer/resources/mojo_private_custom_bindings.js

+// Copyright 2015 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.
+
+/**
+ * Custom bindings for the mojoPrivate API.
+ */
+
+let binding = require('binding').Binding.create('mojoPrivate');

[not at google - send to devlin, 2015/01/16 20:25:14]

I was curious about using "let" over "var", and apparently "let" is behind a
flag that *may* get turned off. So, it's safer to use "var" for now.

+
+binding.registerCustomHook(function(bindingsAPI) {
+  let apiFunctions = bindingsAPI.apiFunctions;
+
+  apiFunctions.setHandleRequest('define', function(name, deps, factory) {
+    define(name, deps || [], factory);
+  });
+});
+
+exports.binding = binding.generate();