Add support for persistent sparse histograms.

base/BUILD.gn

 # Copyright (c) 2013 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.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 518 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 1194 matching lines @@
     "metrics/bucket_ranges_unittest.cc",
     "metrics/field_trial_unittest.cc",
     "metrics/histogram_base_unittest.cc",
     "metrics/histogram_delta_serialization_unittest.cc",
     "metrics/histogram_macros_unittest.cc",
     "metrics/histogram_snapshot_manager_unittest.cc",
     "metrics/histogram_unittest.cc",
     "metrics/metrics_hashes_unittest.cc",
     "metrics/persistent_histogram_allocator_unittest.cc",
     "metrics/persistent_memory_allocator_unittest.cc",
+    "metrics/persistent_sample_map_unittest.cc",
     "metrics/sample_map_unittest.cc",
     "metrics/sample_vector_unittest.cc",
     "metrics/sparse_histogram_unittest.cc",
     "metrics/statistics_recorder_unittest.cc",
     "native_library_unittest.cc",
     "numerics/safe_numerics_unittest.cc",
     "observer_list_unittest.cc",
     "os_compat_android_unittest.cc",
     "path_service_unittest.cc",
     "pickle_unittest.cc",
@@ skipping 438 matching lines @@
 
   # GYP: //base.gyp:base_java_unittest_support
   android_library("base_java_unittest_support") {
     deps = [
       ":base_java",
     ]
     java_files =
         [ "test/android/java/src/org/chromium/base/ContentUriTestUtils.java" ]
   }
 }