base: Cleanup class/struct forward declarations

base/metrics/persistent_histogram_allocator.h

 // Copyright 2016 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_PERSISTENCE_H_
 #define BASE_METRICS_HISTOGRAM_PERSISTENCE_H_
 
 #include <map>
 #include <memory>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/feature_list.h"
 #include "base/memory/shared_memory.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/persistent_memory_allocator.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 
 namespace base {
 
+class BucketRanges;
 class FilePath;
 class PersistentSampleMapRecords;
 class PersistentSparseHistogramDataManager;
 
 // Feature definition for enabling histogram persistence.
 BASE_EXPORT extern const Feature kPersistentHistogramsFeature;
 
 
 // A data manager for sparse histograms so each instance of such doesn't have
 // to separately iterate over the entire memory segment. Though this class
@@ skipping 471 matching lines @@
 
   // The location to which the data should be persisted.
   FilePath persistent_location_;
 
   DISALLOW_COPY_AND_ASSIGN(GlobalHistogramAllocator);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_PERSISTENCE_H_