Add support for persistent sparse histograms.

base/base.gypi

 # 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.
 
 {
   'includes': [
     'trace_event/trace_event.gypi',
   ],
   'target_defaults': {
     'variables': {
@@ skipping 398 matching lines @@
           'metrics/histogram_samples.cc',
           'metrics/histogram_samples.h',
           'metrics/histogram_snapshot_manager.cc',
           'metrics/histogram_snapshot_manager.h',
           'metrics/metrics_hashes.cc',
           'metrics/metrics_hashes.h',
           'metrics/persistent_histogram_allocator.cc',
           'metrics/persistent_histogram_allocator.h',
           'metrics/persistent_memory_allocator.cc',
           'metrics/persistent_memory_allocator.h',
+          'metrics/persistent_sample_map.cc',
+          'metrics/persistent_sample_map.h',
           'metrics/sample_map.cc',
           'metrics/sample_map.h',
           'metrics/sample_vector.cc',
           'metrics/sample_vector.h',
           'metrics/sparse_histogram.cc',
           'metrics/sparse_histogram.h',
           'metrics/statistics_recorder.cc',
           'metrics/statistics_recorder.h',
           'metrics/user_metrics.cc',
           'metrics/user_metrics.h',
@@ skipping 638 matching lines @@
           'i18n/time_formatting.h',
           'i18n/timezone.cc',
           'i18n/timezone.h',
           'i18n/utf8_validator_tables.cc',
           'i18n/utf8_validator_tables.h',
         ],
       }]
     ],
   },
 }