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

mojo/edk/system/channel_endpoint.cc

Index: mojo/edk/system/channel_endpoint.cc
diff --git a/mojo/edk/system/channel_endpoint.cc b/mojo/edk/system/channel_endpoint.cc
index 8d8ecc1558bc877a9423e59ded980b952fa3fb76..6de03d3f65553f9beb624412e5113e7261d4b748 100644
--- a/mojo/edk/system/channel_endpoint.cc
+++ b/mojo/edk/system/channel_endpoint.cc
@@ -7,11 +7,12 @@
 #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;
 
@@ -211,7 +212,7 @@ void ChannelEndpoint::OnReadMessageForClient(
       break;
     }
 
-    base::PlatformThread::YieldCurrentThread();
+    ThreadYield();
   }
 }