Revert of [reland] 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 a0d936b38fe4072aa0b201b22ecb53d0cd6656f2..64f1bbd0bc563bbf3fd67de3d8fc03f76a8dba9d 100644
--- a/base/threading/sequenced_worker_pool.cc
+++ b/base/threading/sequenced_worker_pool.cc
@@ -985,7 +985,7 @@
     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) {