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

mojo/edk/system/test/BUILD.gn

 # 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.
 
 import("../../mojo_edk.gni")
 
 # Utilties for use by EDK internal (unit and perf) tests.
 mojo_edk_source_set("test") {
   testonly = true
   mojo_edk_visibility = [ "mojo/edk/*" ]
 
   sources = [
     "random.cc",
     "random.h",
     "scoped_test_dir.cc",
     "scoped_test_dir.h",
     "simple_test_thread.cc",
     "simple_test_thread.h",
-    "sleep.cc",
-    "sleep.h",
     "stopwatch.cc",
     "stopwatch.h",
     "test_command_line.cc",
     "test_command_line.h",
     "test_io_thread.cc",
     "test_io_thread.h",
     "timeouts.cc",
     "timeouts.h",
   ]
 
@@ skipping 57 matching lines @@
 
   deps = [
     ":test",
     "//base/test:test_support",
   ]
 }
 
 mojo_edk_unittests("mojo_edk_system_test_unittests") {
   sources = [
     "random_unittest.cc",
-    "sleep_unittest.cc",
   ]
 
   deps = [
     ":test",
     "//testing/gtest",
   ]
 }