Remove #pragma once. Just from base/ for now.

base/metrics/field_trial.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.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 56 matching lines @@
 
 // Memory.RendererTotal              // 100% of users still fill this histogram.
 // Memory.RendererTotal_HighMem      // 2% of users will fill this histogram.
 // Memory.RendererTotal_LowMem       // 2% of users will fill this histogram.
 // Memory.RendererTotal_StandardMem  // 96% of users will fill this histogram.
 
 //------------------------------------------------------------------------------
 
 #ifndef BASE_METRICS_FIELD_TRIAL_H_
 #define BASE_METRICS_FIELD_TRIAL_H_
-#pragma once
 
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/observer_list_threadsafe.h"
 #include "base/synchronization/lock.h"
@@ skipping 347 matching lines @@
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_