chrome: Cleanup class/struct forward declarations

chrome/browser/chrome_browser_field_trials_mobile.h

 // Copyright 2013 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 CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_MOBILE_H_
 #define CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_MOBILE_H_
 
 #include "base/time/time.h"
 
-class PrefService;
-
 namespace base {
 class CommandLine;
 }
 
 namespace chrome {
 
 // Sets up mobile-only field trials.
 // Add an invocation of your field trial init function to this method, or to
 // SetupFieldTrials in chrome_browser_field_trials.cc if it is for all
 // platforms.
 void SetupMobileFieldTrials(const base::CommandLine& parsed_command_line);
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_MOBILE_H_