Add //mojo/edk/platform/thread_utils.* containing "yield" and "sleep".

mojo/edk/system/test/sleep.h

-// Copyright 2013 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.
-
-// Function for sleeping in tests. (Please use sparingly.)
-
-#ifndef MOJO_EDK_SYSTEM_TEST_SLEEP_H_
-#define MOJO_EDK_SYSTEM_TEST_SLEEP_H_
-
-#include "mojo/public/c/system/types.h"
-
-namespace mojo {
-namespace system {
-namespace test {
-
-// Sleeps for at least the specified duration.
-void Sleep(MojoDeadline duration);
-void SleepMilliseconds(unsigned duration_milliseconds);
-
-}  // namespace test
-}  // namespace system
-}  // namespace mojo
-
-#endif  // MOJO_EDK_SYSTEM_TEST_SLEEP_H_