Add thread safety to Component updater singleton

chrome/browser/browser_process_impl.cc

 // 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.
 
 #include "chrome/browser/browser_process_impl.h"
 
 #include <algorithm>
 #include <map>
 #include <vector>
 
@@ skipping 767 matching lines @@
 
 prerender::PrerenderTracker* BrowserProcessImpl::prerender_tracker() {
   if (!prerender_tracker_.get())
     prerender_tracker_.reset(new prerender::PrerenderTracker);
 
   return prerender_tracker_.get();
 }
 
 ComponentUpdateService* BrowserProcessImpl::component_updater() {
   if (!component_updater_.get()) {
+    if (!BrowserThread::CurrentlyOn(BrowserThread::UI))
+      return NULL;
     ComponentUpdateService::Configurator* configurator =
         MakeChromeComponentUpdaterConfigurator(
             CommandLine::ForCurrentProcess(),
             io_thread()->system_url_request_context_getter());
     // Creating the component updater does not do anything, components
     // need to be registered and Start() needs to be called.
     component_updater_.reset(ComponentUpdateServiceFactory(configurator));
   }
   return component_updater_.get();
 }
@@ skipping 275 matching lines @@
 }
 
 void BrowserProcessImpl::OnAutoupdateTimer() {
   if (CanAutorestartForUpdate()) {
     DLOG(WARNING) << "Detected update.  Restarting browser.";
     RestartBackgroundInstance();
   }
 }
 
 #endif  // (defined(OS_WIN) || defined(OS_LINUX)) && !defined(OS_CHROMEOS)