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

base/threading/thread_restrictions.h

Index: base/threading/thread_restrictions.h
diff --git a/base/threading/thread_restrictions.h b/base/threading/thread_restrictions.h
index 6ef45dbc6605e8f7d95c409ca01da7705f59706b..8f3beb1d1a24b686e5810dceb9b190d53e345e04 100644
--- a/base/threading/thread_restrictions.h
+++ b/base/threading/thread_restrictions.h
@@ -91,6 +91,7 @@
 }
 
 class SequencedWorkerPool;
+class SimpleThread;
 class Thread;
 class ThreadTestHelper;
 
@@ -189,6 +190,7 @@
   friend class ui::WindowResizeHelperMac;
   friend class MessagePumpDefault;
   friend class SequencedWorkerPool;
+  friend class SimpleThread;
   friend class Thread;
   friend class ThreadTestHelper;
   friend class PlatformThread;