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