Reland r68893 after fixing allowing blocking IO on shutdown.

base/platform_thread_posix.cc

 // Copyright (c) 2006-2008 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.
 
 #include "base/platform_thread.h"
 
 #include <errno.h>
 #include <sched.h>
 
 #include "base/logging.h"
@@ skipping 201 matching lines @@
 bool PlatformThread::CreateNonJoinable(size_t stack_size, Delegate* delegate) {
   PlatformThreadHandle unused;
 
   bool result = CreateThread(stack_size, false /* non-joinable thread */,
                              delegate, &unused);
   return result;
 }
 
 // static
 void PlatformThread::Join(PlatformThreadHandle thread_handle) {
+  // Joining another thread may block the current thread for a long time, since
+  // the thread referred to by |thread_handle| may still be running long-lived /
+  // blocking tasks.
+  base::ThreadRestrictions::AssertIOAllowed();
   pthread_join(thread_handle, NULL);
 }