[Prerender] Get the prerender mode from Finch field trial.

chrome/browser/chrome_browser_field_trials.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 CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_H_
 #define CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_H_
 
-#include "base/command_line.h"
 #include "base/macros.h"
 
 namespace base {
 class FeatureList;
 }
 
 class ChromeBrowserFieldTrials {
  public:
-  explicit ChromeBrowserFieldTrials(const base::CommandLine& command_line);
+  ChromeBrowserFieldTrials();
   ~ChromeBrowserFieldTrials();
 
   void SetupFieldTrials();
 
   // Create field trials that will control feature list features. This should be
   // called during the same timing window as
   // FeatureList::AssociateReportingFieldTrial. |has_seed| indicates that the
   // variations service used a seed to create field trials. This can be used to
   // prevent associating a field trial with a feature that you expect to be
   // controlled by the variations seed.
   void SetupFeatureControllingFieldTrials(bool has_seed,
                                           base::FeatureList* feature_list);
 
  private:
   // Instantiates dynamic trials by querying their state, to ensure they get
   // reported as used.
   void InstantiateDynamicTrials();
 
-  const base::CommandLine& parsed_command_line_;
-
   DISALLOW_COPY_AND_ASSIGN(ChromeBrowserFieldTrials);
 };
 
 #endif  // CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_H_