chrome: Cleanup class/struct forward declarations

chrome/browser/plugins/plugins_field_trial.h

 // Copyright 2015 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_PLUGINS_PLUGINS_FIELD_TRIAL_H_
 #define CHROME_BROWSER_PLUGINS_PLUGINS_FIELD_TRIAL_H_
 
 #include "base/macros.h"
 #include "components/content_settings/core/common/content_settings.h"
 #include "components/content_settings/core/common/content_settings_types.h"
 
-class GURL;
 class HostContentSettingsMap;
 
 // This class manages the Plugins field trials.
 class PluginsFieldTrial {
  public:
   // The name of the variations parameter used for updating the amount of site
   // engagement required to permit Flash.
   static const char* kSiteEngagementThresholdForFlashKey;
 
   // Returns the effective content setting for plugins. Passes non-plugin
   // content settings through without modification.
   static ContentSetting EffectiveContentSetting(
       const HostContentSettingsMap* host_content_settings_map,
       ContentSettingsType type,
       ContentSetting setting);
 
   // Returns the engagement cutoff for permitting Flash to run without a prompt.
   static double GetSiteEngagementThresholdForFlash();
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PluginsFieldTrial);
 };
 
 #endif  // CHROME_BROWSER_PLUGINS_PLUGINS_FIELD_TRIAL_H_