base: Cleanup class/struct forward declarations

base/metrics/histogram_base.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 BASE_METRICS_HISTOGRAM_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/strings/string_piece.h"
 #include "base/time/time.h"
 
 namespace base {
 
-class BucketRanges;
 class DictionaryValue;
 class HistogramBase;
 class HistogramSamples;
 class ListValue;
 class Pickle;
 class PickleIterator;
 
 ////////////////////////////////////////////////////////////////////////////////
 // This enum is used to facilitate deserialization of histograms from other
 // processes into the browser. If you create another class that inherits from
@@ skipping 232 matching lines @@
 
   const std::string histogram_name_;
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_