Revert of Do not block in SimpleThread::Start().

base/threading/sequenced_worker_pool.cc

Index: base/threading/sequenced_worker_pool.cc
diff --git a/base/threading/sequenced_worker_pool.cc b/base/threading/sequenced_worker_pool.cc
index be483bc7e2d0d0a09f4ab29a5e8d075702ba25aa..badd2936ee2a70b964b5d047c85a5c8ce0861925 100644
--- a/base/threading/sequenced_worker_pool.cc
+++ b/base/threading/sequenced_worker_pool.cc
@@ -977,7 +977,7 @@
     DCHECK(thread_being_created_);
     thread_being_created_ = false;
     auto result = threads_.insert(
-        std::make_pair(PlatformThread::CurrentId(), WrapUnique(this_worker)));
+        std::make_pair(this_worker->tid(), WrapUnique(this_worker)));
     DCHECK(result.second);
 
     while (true) {