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

base/metrics/histogram.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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 
 // It supports calls to accumulate either time intervals (which are processed
 // as integral number of milliseconds), or arbitrary integral units.
@@ skipping 21 matching lines @@
 // pointer to a histogram.  This static is explicitly initialized on any thread
 // that detects a uninitialized (NULL) pointer.  The potentially racy
 // initialization is not a problem as it is always set to point to the same
 // value (i.e., the FactoryGet always returns the same value).  FactoryGet
 // is also completely thread safe, which results in a completely thread safe,
 // and relatively fast, set of counters.  To avoid races at shutdown, the static
 // pointer is NOT deleted, and we leak the histograms at process termination.
 
 #ifndef BASE_METRICS_HISTOGRAM_H_
 #define BASE_METRICS_HISTOGRAM_H_
-#pragma once
 
 #include <list>
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/gtest_prod_util.h"
@@ skipping 840 matching lines @@
   // of our data, and to quickly see if some other CachedRanges instance is
   // possibly Equal() to this instance.
   uint32 range_checksum_;
 
   DISALLOW_COPY_AND_ASSIGN(CachedRanges);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_H_