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

base/threading/thread_restrictions.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_THREADING_THREAD_RESTRICTIONS_H_
 #define BASE_THREADING_THREAD_RESTRICTIONS_H_
 
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/macros.h"
@@ skipping 73 matching lines @@
 
 namespace android {
 class JavaHandlerThread;
 }
 
 namespace internal {
 class TaskTracker;
 }
 
 class SequencedWorkerPool;
+class SimpleThread;
 class Thread;
 class ThreadTestHelper;
 
 // Certain behavior is disallowed on certain threads.  ThreadRestrictions helps
 // enforce these rules.  Examples of such rules:
 //
 // * Do not do blocking IO (makes the thread janky)
 // * Do not access Singleton/LazyInstance (may lead to shutdown crashes)
 //
 // Here's more about how the protection works:
@@ skipping 78 matching lines @@
   friend class ::HistogramSynchronizer;
   friend class internal::TaskTracker;
   friend class ::ScopedAllowWaitForLegacyWebViewApi;
   friend class cc::CompletionEvent;
   friend class cc::SingleThreadTaskGraphRunner;
   friend class content::CategorizedWorkerPool;
   friend class remoting::AutoThread;
   friend class ui::WindowResizeHelperMac;
   friend class MessagePumpDefault;
   friend class SequencedWorkerPool;
+  friend class SimpleThread;
   friend class Thread;
   friend class ThreadTestHelper;
   friend class PlatformThread;
   friend class android::JavaHandlerThread;
   friend class mojo::SyncCallRestrictions;
   friend class mojo::edk::ScopedIPCSupport;
   friend class ui::CommandBufferClientImpl;
   friend class ui::CommandBufferLocal;
   friend class ui::GpuState;
 
@@ skipping 42 matching lines @@
 
     DISALLOW_COPY_AND_ASSIGN(ScopedAllowWait);
   };
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadRestrictions);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_RESTRICTIONS_H_