Consider PlatformThread::Join() to be blocking IO.

base/platform_thread_win.cc

 // Copyright (c) 2010 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 "base/logging.h"
 #include "base/thread_restrictions.h"
 #include "base/win/windows_version.h"
 
@@ skipping 107 matching lines @@
 }
 
 // static
 bool PlatformThread::CreateNonJoinable(size_t stack_size, Delegate* delegate) {
   return CreateThreadInternal(stack_size, delegate, NULL);
 }
 
 // static
 void PlatformThread::Join(PlatformThreadHandle thread_handle) {
   DCHECK(thread_handle);
+  base::ThreadRestrictions::AssertIOAllowed();
 
   // Wait for the thread to exit.  It should already have terminated but make
   // sure this assumption is valid.
   DWORD result = WaitForSingleObject(thread_handle, INFINITE);
   DCHECK_EQ(WAIT_OBJECT_0, result);
 
   CloseHandle(thread_handle);
 }