Get rid of our own silly reimplementation of std::thread

ext/native/thread/prioritizedworkqueue.cpp

@@ -1,8 +1,8 @@
 #include <functional>
+#include <thread>
 
 #include "base/logging.h"
 #include "base/timeutil.h"
-#include "thread/thread.h"
 #include "thread/prioritizedworkqueue.h"
 
 PrioritizedWorkQueue::~PrioritizedWorkQueue() {