Add some closureised JS bindings for DevTools for use by headless embedder

headless/lib/tab_socket_externs.js

Index: headless/lib/tab_socket_externs.js
diff --git a/headless/lib/tab_socket_externs.js b/headless/lib/tab_socket_externs.js
new file mode 100644
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+// Copyright 2017 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.
+
+/**
+ * @fileoverview Headless TabSocket externs.
+ * @externs
+ */
+
+window.TabSocket = {};
+window.TabSocket.send = function(a) {};
+window.TabSocket.onmessage = {};