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

mojo/edk/system/channel_endpoint.cc

 // Copyright 2014 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.
 
 #include "mojo/edk/system/channel_endpoint.h"
 
 #include <utility>
 
 #include "base/logging.h"
-#include "base/threading/platform_thread.h"
+#include "mojo/edk/platform/thread_utils.h"
 #include "mojo/edk/system/channel.h"
 #include "mojo/edk/system/channel_endpoint_client.h"
 #include "mojo/public/cpp/system/macros.h"
 
+using mojo::platform::ThreadYield;
 using mojo::util::MutexLocker;
 using mojo::util::RefPtr;
 
 namespace mojo {
 namespace system {
 
 bool ChannelEndpoint::EnqueueMessage(
     std::unique_ptr<MessageInTransit> message) {
   DCHECK(message);
 
@@ skipping 179 matching lines @@
       // Take a ref, and call |OnReadMessage()| outside the lock.
       client = client_;
       client_port = client_port_;
     }
 
     if (client->OnReadMessage(client_port, message.get())) {
       ignore_result(message.release());
       break;
     }
 
-    base::PlatformThread::YieldCurrentThread();
+    ThreadYield();
   }
 }
 
 void ChannelEndpoint::DieNoLock() {
   DCHECK(state_ == State::RUNNING);
   DCHECK(channel_);
   DCHECK(local_id_.is_valid());
   DCHECK(remote_id_.is_valid());
 
   state_ = State::DEAD;
   channel_ = nullptr;
   local_id_ = ChannelEndpointId();
   remote_id_ = ChannelEndpointId();
 }
 
 }  // namespace system
 }  // namespace mojo