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

mojo/edk/system/core_unittest.cc

Index: mojo/edk/system/core_unittest.cc
diff --git a/mojo/edk/system/core_unittest.cc b/mojo/edk/system/core_unittest.cc
index 18f3fda72a587368bcbab71067a8624662ad8463..268aadbdf6ece126b65fd0c45da3fb66bae45c0d 100644
--- a/mojo/edk/system/core_unittest.cc
+++ b/mojo/edk/system/core_unittest.cc
@@ -8,11 +8,14 @@
 
 #include <limits>
 
+#include "mojo/edk/platform/thread_utils.h"
 #include "mojo/edk/system/awakable.h"
 #include "mojo/edk/system/core_test_base.h"
-#include "mojo/edk/system/test/sleep.h"
+#include "mojo/edk/system/test/timeouts.h"
 #include "mojo/public/cpp/system/macros.h"
 
+using mojo::platform::ThreadSleep;
+
 namespace mojo {
 namespace system {
 namespace {
@@ -29,7 +32,7 @@ TEST_F(CoreTest, GetTimeTicksNow) {
   const MojoTimeTicks start = core()->GetTimeTicksNow();
   EXPECT_NE(static_cast<MojoTimeTicks>(0), start)
       << "GetTimeTicksNow should return nonzero value";
-  test::SleepMilliseconds(15u);
+  ThreadSleep(test::DeadlineFromMilliseconds(15u));
   const MojoTimeTicks finish = core()->GetTimeTicksNow();
   // Allow for some fuzz in sleep.
   EXPECT_GE((finish - start), static_cast<MojoTimeTicks>(8000))