EDK: Add a mojo::platform::MessageLoop abstraction.

mojo/edk/platform/test_message_loop.h

+// 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.
+
+// This file declares a factory function (which must be implemented by the
+// embedder) for message loops to be used in tests.
+
+#ifndef MOJO_EDK_PLATFORM_TEST_MESSAGE_LOOP_H_
+#define MOJO_EDK_PLATFORM_TEST_MESSAGE_LOOP_H_
+
+#include <memory>
+
+#include "mojo/edk/platform/message_loop.h"
+
+namespace mojo {
+namespace platform {
+namespace test {
+
+// Creates a basic |platform::MessageLoop|, to be used by tests. This must be
+// implemented by the embedder (if building tests that require it).
+std::unique_ptr<MessageLoop> CreateTestMessageLoop();
+
+}  // namespace test
+}  // namespace platform
+}  // namespace mojo
+
+#endif  // MOJO_EDK_PLATFORM_TEST_MESSAGE_LOOP_H_