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

mojo/edk/platform/BUILD.gn

Index: mojo/edk/platform/BUILD.gn
diff --git a/mojo/edk/platform/BUILD.gn b/mojo/edk/platform/BUILD.gn
index f03b7123c7f5db1383f9df50765ebf979a77c868..3a30b7fce339490015f67fac72c3c40977225a3f 100644
--- a/mojo/edk/platform/BUILD.gn
+++ b/mojo/edk/platform/BUILD.gn
@@ -18,6 +18,7 @@ mojo_edk_source_set("platform") {
     "scoped_platform_handle.h",
     "task_runner.h",
     "thread.h",
+    "thread_utils.h",
   ]
 
   mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
@@ -59,6 +60,7 @@ mojo_edk_source_set("unittests") {
     "aligned_alloc_unittest.cc",
     "io_thread_unittest.cc",
     "test_message_loops_unittest.cc",
+    "thread_utils_unittest.cc",
   ]
 
   deps = [
@@ -67,5 +69,10 @@ mojo_edk_source_set("unittests") {
     "//testing/gtest",
   ]
 
+  # TODO(vtl): This is a suboptimal dependency. Probably "Stopwatch" should be
+  # moved to :test_platform (which we can do after we add a clock/time function
+  # to :platform).
+  mojo_edk_deps = [ "mojo/edk/system/test" ]
+
   mojo_sdk_deps = [ "mojo/public/cpp/system" ]
 }