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

chrome/browser/browser_process_impl.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 "chrome/browser/browser_process_impl.h"
 
 #include <map>
 
 #include "app/clipboard/clipboard.h"
 #include "app/l10n_util.h"
 #include "base/command_line.h"
 #include "base/file_util.h"
 #include "base/path_service.h"
 #include "base/task.h"
 #include "base/thread.h"
+#include "base/thread_restrictions.h"
 #include "base/waitable_event.h"
 #include "chrome/browser/appcache/chrome_appcache_service.h"
 #include "chrome/browser/automation/automation_provider_list.h"
 #include "chrome/browser/browser_child_process_host.h"
 #include "chrome/browser/browser_list.h"
 #include "chrome/browser/browser_main.h"
 #include "chrome/browser/browser_process_sub_thread.h"
 #include "chrome/browser/browser_thread.h"
 #include "chrome/browser/browser_trial.h"
 #include "chrome/browser/debugger/debugger_wrapper.h"
@@ skipping 217 matching lines @@
   did_start_ = true;
   module_ref_count_++;
   return module_ref_count_;
 }
 
 unsigned int BrowserProcessImpl::ReleaseModule() {
   DCHECK(CalledOnValidThread());
   DCHECK_NE(0u, module_ref_count_);
   module_ref_count_--;
   if (0 == module_ref_count_) {
+    // Allow UI and IO threads to do blocking IO on shutdown, since we do a lot
+    // of it on shutdown for valid reasons.
+    base::ThreadRestrictions::SetIOAllowed(true);
+    io_thread()->message_loop()->PostTask(
+        FROM_HERE,
+        NewRunnableFunction(&base::ThreadRestrictions::SetIOAllowed, true));
     MessageLoop::current()->PostTask(
         FROM_HERE, NewRunnableFunction(DidEndMainMessageLoop));
     MessageLoop::current()->Quit();
   }
   return module_ref_count_;
 }
 
 void BrowserProcessImpl::EndSession() {
 #if defined(OS_WIN) || defined(USE_X11)
   // Notify we are going away.
@@ skipping 553 matching lines @@
 }
 
 void BrowserProcessImpl::OnAutoupdateTimer() {
   if (CanAutorestartForUpdate()) {
     DLOG(WARNING) << "Detected update.  Restarting browser.";
     RestartPersistentInstance();
   }
 }
 
 #endif  // (defined(OS_WIN) || defined(OS_LINUX)) && !defined(OS_CHROMEOS)