Remove direct usage of BrowserThreadImpl in tests

content/browser/browser_thread_impl.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 CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_
 #define CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_
 
 #include "base/threading/platform_thread.h"
 #include "base/threading/thread.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/browser_thread.h"
 
 namespace content {
 
+// Very few users should use this directly. To mock BrowserThreads, tests should
+// use TestBrowserThreadBundle instead.
 class CONTENT_EXPORT BrowserThreadImpl : public BrowserThread,
                                          public base::Thread {
  public:
   // Construct a BrowserThreadImpl with the supplied identifier.  It is an error
   // to construct a BrowserThreadImpl that already exists.
   explicit BrowserThreadImpl(BrowserThread::ID identifier);
 
   // Special constructor for the main (UI) thread and unittests. If a
   // |message_loop| is provied, we use a dummy thread here since the main
   // thread already exists.
@@ skipping 44 matching lines @@
   static void FlushThreadPoolHelperForTesting();
 
   // The identifier of this thread.  Only one thread can exist with a given
   // identifier at a given time.
   ID identifier_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BROWSER_THREAD_IMPL_H_