Reland "EDK: Move //mojo/edk/system/waitable_event* to edk/util."

mojo/edk/system/test/test_io_thread.h

Index: mojo/edk/system/test/test_io_thread.h
diff --git a/mojo/edk/system/test/test_io_thread.h b/mojo/edk/system/test/test_io_thread.h
index 8b0d6e39e952aff5cb2f247a89519d970bbc96ef..571c7909c16890ecb9146bc2d8355db57a6d3fab 100644
--- a/mojo/edk/system/test/test_io_thread.h
+++ b/mojo/edk/system/test/test_io_thread.h
@@ -29,10 +29,10 @@ class TestIOThread {
   void Start();
   void Stop();
 
-  // Post |task| to the IO thread.
+  // Posts |task| to the I/O thread.
   void PostTask(const base::Closure& task);
-  // Posts |task| to the IO-thread with an WaitableEvent associated blocks on
-  // it until the posted |task| is executed, then returns.
+  // Posts |task| to the I/O thread, blocking the calling thread until the
+  // posted task is executed (note the deadlock risk!).
   void PostTaskAndWait(const base::Closure& task);
 
   base::MessageLoopForIO* message_loop() {